refactor: add data subdirectory and mountpoint directory to the global config (#159)

Description

Allow having nested subdirectories for PGDATA relative to the pool mount directory:

• add data subdirectory
• add mountpoint directory to the global config

Related issue

#159 (closed)

Examples

global:
  mountDir: "/var/lib/dblab"
  dataSubDir: "data"

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes, specifications reflect those changes and they have been reviewed OR there are no API changes

Closes #159 (closed)