First steps in consolidating attributes postgresql['dir'] and postgresql['data_dir']"

Related to #2214 (closed)

This makes a few postgresql attributes relative to postgresql['dir']. Allowing:

1. The database can be run from a different directory
2. /var/opt/gitlab/postgresql/data is no longer required to always exist
3. postgresql['data_dir'] and postgresql['dir'] can point to completely separate directories without issue

This is a pre-requisite to potentially deprecating postgresql['data_dir']