Remove _is_true() from test_volumes_actions.py
Remove _is_true() from test_volumes_actions.py

This commit removes the _is_true() function. Because APIs should be
stable, so we shouldn't need like this function for normalizing values.
Otherwise, API users also need something like that function.

Change-Id: Idbd9780e8fd71d2e1ba399336333c22485b18d93

vedujoshi/tempest,Juniper/tempest,masayukig/tempest,masayukig/tempest,openstack/tempest,vedujoshi/tempest,cisco-openstack/tempest,openstack/tempest,cisco-openstack/tempest,Juniper/tempest