engine: Refactoring AddVdsCommand#canDoAction()

to use failCanDoAction(), saving few lines of code. Also made sure that
the gluster specific validation happens only if all previous validation
was successful.

Change-Id: Id2525aa9ce72654fea18eb60948a87d95a636602
Signed-off-by: Shireesh Anjal <anjalshireesh@gmail.com>