engine: fix locking issue in GlusterMultipleActionsRunner

GlusterMultipleActionsRunner#executeValidatedCommands() had code to
re-acquire the lock released at the end of canDoAction. However, this
should not happen in case the CanDoAction had failed, as
MultipleActionsRunner#executeValidatedCommand() doesn't even execute
the command in this case.

Performed a minor refactoring in MultipleActionsRunner as follows:
- renamed executeValidatedCommands() to executeValidatedCommand()
- executeValidatedCommand() gets called only when canDoAction was success

Change-Id: I100603d7155deb67bb2f0a8b451732a0003fbf55
Bug-Url: https://bugzilla.redhat.com/905904
Signed-off-by: Shireesh Anjal <sanjal@redhat.com>