an improved version of PR #696

- remove configuration of OpenDKIM
- remove OpenDKIM milter from postfix's configuration
- add configuration files for rpsamd's dkim module
- update the rspamd task
- update services in README

- Remove OpenDMARC
- Configure Rspamd for DMARC handling
- Update services and how to set up DNS records in README

Rmilter [is no longer
needed](https://rspamd.com/doc/quickstart.html#rmilter-setup) as of
Rspamd 1.6.

The configuration file `90-plugin.conf` exists but was not getting
installed.

there is a missing semicolon that causes rmilter not to start with error:
"config file parse error! line: 156, text: tempdir, reason: syntax error"

which, as expected is the following line

The stock rspamd package provided by Raspbian (jessie) is at version 0.6.10.  The rspamd repository doesn't offer binaries for the armhf architecture (e.g., Raspberry Pi 3) in order to get a more recent package.  Instead, use the yunohost repository for rspamd package (currently at 1.3.5) on armhf.

without this change it fails on ansible(2.2.0.0) with the following message:

    TASK [mailserver : Generate OpenDKIM keys] *************************************
    fatal: [sovereign.host]: FAILED! => {"failed": true, "msg": "the field 'args' has an invalid value, which appears to include a variable that is undefined. The error was: 'unicode object' has no attribute 'name'\n\nThe error appears to have been in '/Users/nfedyashev/Projects/sovereign/roles/mailserver/tasks/opendkim.yml': line 19, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Generate OpenDKIM keys\n  ^ here\n"}

Newer versions of Ansible complain when using bare variables like
`mail_virtual_users` and prefer the newer `'{{ mail_virtual_users }}'`
syntax.

Noticed the depreciation warning with Ansible version 2.1.2.0.

Use a Unix socket instead of a TCP socket as a Unix socket doesn't play nicely with postfix running
smtpd in a chroot. The author of rmilter recommends using a TCP socket per
https://github.com/vstakhov/rmilter/issues/39

Postgres is used by several roles, but the setup is currently part of the 'mailserver' role. By moving it to 'common', it's possible to disable the mailserver without breaking the others.

Instead of registering a handler to run when the database is created,
register a variable and check it immediately to run the schema import.
This avoids a problem where an error between database creation and
schema import 1) leaves the server in a broken state, and 2) rerunning
the playbook doesn't fix it.

Change the name from `dmarc` to `opendmarc` to be consistent with the
names of other task lists in the role.

Clean up some artifacts from the merge to match the `jessie` branch.
Keep the changes on master made with commits bfe6fe84 and e229c551.

The rmilter package changed the name of its listener service similar to
what was done for rspamd (see commit 2dbc976b).

Currently client email is submitted via ssmtp (port 465).  This has been
deprecated for years.  The correct way to submit email is via
submission (port 587).

This patch adds port 587 as a second and the default way of submitting
email for delivery.  Port 465 remains open for backwards compatibility
with existing clients.

This is done to suppress warnings from ansible-lint.

An earlier commit started transitioning opendmarc to use postgres, but
this was incomplete.  This patch reverts that change and uses mysql for
the reporting database.

Other changes:

* Do not maintain a copy of the database import schema.  A copy is
  included in the distribution in /usr/share/doc, so that is used
  instead.
* The configuration file is replaced with the distribution's sample
  configuration.  A second patch will restore the actual configuration.
  This will make the changes easier to see if the default configuraton
  file changes in future versions of opendmarc.

Make a clean distinction between Debian 7 and Debian 8.  Anticipate the
next Ubuntu LTS release (Xenial) that is planned for support.