nextcloud: Support remote database server

The _nextcloud_ role originally handled setting up the PostgreSQL
database and assumed that it was running on the same server as Nextcloud
itself.  I have factored out those tasks into their own role,
_nextcloud-db_, which can be applied to a separate host.

I have also introduced some new variables (`nextcloud_db_host`,
`nextcloud_db_name`, `nextcloud_db_user`, and `nextcloud_db_password`),
which can be used to specify how to connect to the database, if it is
hosted remotely.  Since these variables are used by both the _nextcloud_
and _nextcloud-db_ roles, they are actually defined in a separate role,
_nextcloud-base_, upon which both depend.

roles/nextcloud/templates/config.php.j2

@@ -37,12 +37,12 @@ $CONFIG = array (
   'dbtype' => 'pgsql',
   'version' => '{{ nc_version }}',
   'overwrite.cli.url' => 'https://{{ nextcloud_server_name }}',
-  'dbname' => 'nextcloud',
-  'dbhost' => 'localhost',
-  'dbport' => '',
+  'dbname' => '{{ nextcloud_db_name }}',
+  'dbhost' => '{{ nextcloud_db_host }}',
+  'dbport' => '{{ nextcloud_db_port }}',
   'dbtableprefix' => 'oc_',
-  'dbuser' => 'nextcloud',
-  'dbpassword' => '{{ nextcloud_db_password }}',
+  'dbuser' => '{{ nextcloud_db_user }}',
+  'dbpassword' => '{{ nextcloud_db_password|d("") }}',
   'installed' => true,
   'ldapIgnoreNamingRules' => false,
   'ldapProviderFactory' => 'OCA\\User_LDAP\\LDAPProviderFactory',