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)
Fixes missing ec2 api address disassociate error on failure

Fixes ec2 api address disassociate so that the ec2 error
InvalidAssociationID.NotFound is returned when it is passed an
invalid association rather than returning success. Note this is
slightly different than the original bug report but the type
of failure has changed since the bug was originally filed.

Change-Id: I10034a2fed62d4e93bc384aef3ff594b66e285f2
Closes-Bug: 1179816
(cherry picked from commit 00d8c334bbfaef3ffec4b04b494e609740b96253)
Merge "Fixes missing ec2 api address disassociate error on failure" into stable/icehouse
Merge "Ignore errors when deleting non-existing vifs" into stable/icehouse