Add a config variable to configure the time for the 'when_to_delete' and 'minimun_age_to_delete' variables

Problem/Motivation

At this moment this two values are stored in two constant, but is better if the user can configure this values by himself.

```
define('NODE_REVISION_DELETE_WHEN_TO_DELETE_MAXIMUN_MONTHS', 12);
define('NODE_REVISION_DELETE_MINIMUN_AGE_TO_DELETE_MAXIMUN_MONTHS', 12);

```

Proposed resolution

Create two variables in the settings files named:

```
node_revision_delete_when_to_delete_time
node_revision_delete_minimun_age_to_delete_time

```

And allow to select between days, weeks and months.

Remaining tasks

None.

User interface changes

Two new variables to config from the module config page.

API changes

None.

Data model changes

The settings and schema files will be changed.