Don't send heartbeats if Manager reports a problem

Don't send heartbeats if Manager reports a problem

This commit adds is_working method to base Manager class that can be
used to indicate that service has some problems and isn't running
correctly. This is used to block refreshing Service heartbeats if
Manager will return False from is_working.

The method is implemented in c-vol Manager to return initialization
state of the underlying volume driver.

Closes-Bug: 1446750
DocImpact

Change-Id: I809b2b777a0f84d12f39f2233920ec961f74f846