Resolves #266.

Subsequent runs of wallabag's playbook result in the 'install' directory being
recreated, (after the "Clone wallabag" step) kicking wallabag's first time
setup procedure.

With this patch we automatically remove the 'install' directory if wallabag's
configuration file appears to be in place before cloning the repository.

/etc/apache2/sites-available and old (dangling) symlinks in
/etc/apaches/sites-enabled.
Also, remove unnecessary apache2 restart after creating a new
virtualhost but not yet enabling it.

The virtual site files must be owned by root (serious security issue)
and they must have the .conf filename suffix for a2ensite on
Ubuntu 14.04LTS (apache 2.4.7).