Remove api = apiName calls from each method

As discussed in https://review.openstack.org/#/c/36352/ for each
command, we were setting api = identity or volume... etc,
this was for an old way of calling commands that are is no longer
used.

Also removed openstackclient/common/command.py

Change-Id: I2705f35d343f2ae729dc22d6aed0b852b2f8ca19
Merge branch 'image-v1' into dt-test
Merge branch 'identity-catalog' into dt-test
Merge branch 'limits-show' into dt-test
Merge branch 'volume-keystone' into dt-test