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.

nextcloud.yml

@@ -1,7 +1,14 @@
+- hosts: nextcloud-db
+  vars_files:
+  - vault/nextcloud
+  roles:
+  - role: nextcloud-db
+    tags:
+    - nextcloud-db
+
 - hosts: nextcloud
   vars_files:
   - vault/nextcloud
   roles:
-  - postgresql-server
   - apache
   - nextcloud