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/tasks/main.yml

@@ -27,21 +27,6 @@
   tags:
   - always
 
-- name: ensure nextcloud database user exists
-  become: true
-  become_user: postgres
-  postgresql_user:
-    name: nextcloud
-    password: '{{ nextcloud_db_password }}'
-    state: present
-- name: ensure nextcloud database exists
-  become: true
-  become_user: postgres
-  postgresql_db:
-    name: nextcloud
-    owner: nextcloud
-    state: present
-
 - name: ensure nextcloud is configured
   template:
     src: config.php.j2