Bug #1086232
We introduce the action=clear to unknown argument part of
the command line. With this option, user can specify which
attribute will have None value.
For example to set dns_nameservers of subnet to None, we use:
quantum subnet-update subnetname --dns_nameservers action=clear
Of course, client command developer can use known option like --no-gateway
to do the same job.
Change-Id: Ia8dffcfd1122259d545a61e71db25e459639d130
f9ca01c81e