Ignore errors when deleting non-existing vifs

If an instance fails during its network creation (for example
if the network-vif-plugged event doesn't arrive in time) a
subsequent delete will also fail when it tries to delete the
vif, leaving the instance in a Error(deleting) state.

This can be avoided by including the "--if-exists" option to
the ovs-vsctl command so that the command doesn't fail if the
vif doesn't exist.

Change-Id: Ic613c9d6e0619f51fcd931e62f6d6bce404ebe81
Closes-bug: #1308544
(cherry picked from commit 476dc5efcdb11c9859b062dc69a8c205e69be861)
Merge "Ignore errors when deleting non-existing vifs" into stable/icehouse