Add a config variable to configure the minimum_revisions_to_keep variable

Problem/Motivation

We need to define a minimum number of revision to keep for each content type, this quantity must be in the system even if the revisions are older than the 'Minimum age of revision to delete'.

Proposed resolution

Create a new variable named minimum_revisions_to_keep inside the node_revision_delete_track array.

Remaining tasks

None.

User interface changes

A new variable to config from the module config page.

API changes

None.

Data model changes

The settings and schema files will be changed.
The revisions_to_keep variable will be replaced by this new variable.