Drush command to configure how many revisions delete per cron run

Problem/Motivation

You can configure by the user interface how many revisions delete per cron run, but this is not possible through a drush command.

Proposed resolution

We need something like this:

Delete 50 revisions per cron run:

`drush nrd-delete-cron-run 50`

Remaining tasks

None.

User interface changes

None.

API changes

Added the new drush command:

`drush nrd-delete-cron-run`

Data model changes

None.