Allow user to specify None value to attributes

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
Merge "Allow user to specify None value to attributes"