Neutron tag mechanism now supports network, subnet, port,
subnetpool and router. Tag support for more resources is planned.
This commit introduces a common mixin class to implement
tag operation and individual resource consumes it.
To support tag remove, network unset command is added.
Implements blueprint neutron-client-tag
Change-Id: Iad59d052f46896d27d73c22d6d4bb3df889f2352