On adding a tag to a resource (port/subnet/subnetpool/network),
if the resource already has the tag, skip the neutron API call to
add_tag because the tags will be unchanged.
This will avoid one or two Neutron API calls and optimize the
performance a bit.
Partial-Bug: #1809306
Change-Id: I8a181d6af5228007d36fa8b217fec7f766dd37fd