Remove "--disabled" for firewall create rule

Allow "--enabled True/False" and remove "--disabled" for consistency across
all commands. Even in other neutron commands, we are using only --enabled
option but not --disabled. firewall-rule-show displays only "enabled" attribute
and there is no "disabled" attribute.

Change-Id: Ie591d827660466a774e431dcb40e04e09773e5f7
Closes-bug: 1327056
Merge "Remove "--disabled" for firewall create rule"