Description of problem:
When SBD with no disks is enabled in a cluster with even numerb of nodes, auto_tie_breaker must be enabled in orded for SBD to work correctly. Pcs should prevent disabling auto_tie_breaker in this case.

Version-Release number of selected component (if applicable):
pcs-0.11.6

How reproducible:
always, easily

Steps to Reproduce:
1. create a cluster with two nodes
2. enable SBD: 'pcs stonith sbd enable'
3. try to disable ATB:

Actual results:
In step 2, pcs informs the user that ATB must be enabled. Yet, in step 3, pcs is fine with disabling ATB.

Expected results:
Pcs prevents disabling SBD in step 3.