Use Enum value instead of String Value

[David TARDIVEL]

In the Watcher source code, we have again some code like this:

https://github.com/openstack/watcher/blob/master/watcher/objects/action.py#L201

        self.state = "DELETED"

or

https://github.com/openstack/watcher/blob/master/watcher/tests/db/test_audit.py#L274

        audit2 = self._create_test_audit(
            id=2,
            audit_type='CONTINUOUS',
            uuid=w_utils.generate_uuid(),
            deadline=None,
            state='PENDING')

String value should be replace by Enum related one.

in our previous example, we should have:

        self.state = State.DELETED
or

        audit2 = self._create_test_audit(
            id=2,
            audit_type=objects.audit.AuditType.CONTINUOUS.value,
            uuid=w_utils.generate_uuid(),
            deadline=None,
            state=objects.audit.State.PENDING)

[Atul Pandey (atul-4you)]

Submitted the review Patch : https://review.openstack.org/#/c/390440/

[Atul Pandey (atul-4you)]

Patch merged : https://review.openstack.org/#/c/390440/