Add DRBD support for HA

This is very similar to what was done for the database barclamp:
https://github.com/crowbar/barclamp-database/pull/67

Co-Authored-By: Vincent Untz <vuntz@suse.com>
Add ability to choose size of DRBD device

Since it's a LVM volume, we can't guess what's right and it's better to
leave it up to the user.