core: Allow skipping vm command execution

This patch adds an easy way to skip vm command execution,
by overriding 'shouldSkipCommandExecution' method,
we can mark the	command not to run can-do-action and execute code.
the command will just finish as succeeded and operation will be logged.

in change-id: I75925d528cca52603660d5a7ec6c5c871304f3d0
there is an example of usage for the 'stop down vm' scenario.

Change-Id: I290055f759afcc554d1f49fac12455ae18d2d194
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1168304
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>