engine: Set revert operation scope for ImportVMTemplate command

Setting this scope will impose ImportVMTemplate command to perform
revert in case of failure. Without revert DB continue to keep leftovers
from previous run so user can't re-import the same template without
cloning it.

Change-Id: Ic1d8220f2b4934dc9473d2ad3c4f7ceb4623e3a7
Bug-Url: https://bugzilla.redhat.com/1005788
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>