make NTP pool configurable

use the world-wide pool by default, but specify north-america in
user.yml. Also, documentation. This way Sovereign will still behave the
same, but the NTP servers can be changed when desired.

roles/common/templates/ntp.conf.j2

 filegen peerstats file peerstats type day enable
 filegen clockstats file clockstats type day enable
 
-# Use servers from the NTP Pool Project
-server 0.north-america.pool.ntp.org
-server 1.north-america.pool.ntp.org
-server 2.north-america.pool.ntp.org
-server 3.north-america.pool.ntp.org
+# Use servers configured via Ansible
+{% for server in ntp_servers %}
+server {{ server }}
+{% endfor %}
 
 # fallback
 server tick.usno.navy.mil

vars/defaults.yml

 friendly_networks:
   - ""
 
+# ntp
+ntp_servers:
+  # use a random selection of worldwide servers by default.
+  - 0.pool.ntp.org
+  - 1.pool.ntp.org
+  - 2.pool.ntp.org
+  - 3.pool.ntp.org
+
 # database
 db_admin_username: 'postgres'
 # db_admin_password: (required)

vars/testing.yml

 db_admin_username: postgres
 db_admin_password: postgres
 
+ntp_servers:
+  - 0.north-america.pool.ntp.org
+  - 1.north-america.pool.ntp.org
+  - 2.north-america.pool.ntp.org
+  - 3.north-america.pool.ntp.org
+
 # ircbouncer
 irc_nick: sovereign
 irc_ident: sovereign

vars/user.yml

 main_user_name: TODO
 encfs_password: TODO
 
+# ntp
+ntp_servers:
+  # preferably, use the ntp servers in your own country or continent
+  # See http://www.pool.ntp.org/en/use.html
+  - 0.north-america.pool.ntp.org
+  - 1.north-america.pool.ntp.org
+  - 2.north-america.pool.ntp.org
+  - 3.north-america.pool.ntp.org
+
 # database
 db_admin_username: postgres
 db_admin_password: TODO