crypto: Move USER_CREATABLE to secret_common base class

Instead of duplicating the code for user creatable objects in secret and
secret_keyring, move it to the common base clase secret_common. As the
base class is abstract, it won't become user creatable itself.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>