Add application_credential module

Create or delete a Keystone application credential.  When the secret
parameter is not set a secret will be generated and returned in the
response. Existing credentials cannot be modified so running this module
against an existing credential will result in it being deleted and
recreated. This needs to be taken into account when the secret is
generated, as the secret will change on each run of the module.

The returned result also includes a usable cloud config which allows
playbooks to easily run openstack tasks using the credential created by
this module.

Change-Id: I0ed86dc8785b0e9d10cc89cd9137a11d02d03945

ci/roles/application_credential/defaults/main.yml

@@ -0,0 +1,9 @@
+expected_fields:
+  - description
+  - expires_at
+  - id
+  - name
+  - project_id
+  - roles
+  - secret
+  - unrestricted