Ensure the ec2 API supports HEAD
Ensure the ec2 API supports HEAD

This commit makes it so all GET APIs within the ec2 API
support HEAD. This change doesn't include any tests because
the ec2 API is untested, see bug 1635389 for more details.

Change-Id: Iab7326d0758425f535c57c64f9cdca5f2e8d122b
Partial-Bug: 1696574

openstack/keystone,mahak/keystone,openstack/keystone,mahak/keystone,mahak/keystone,openstack/keystone