Explicitly name element enable-serial-console

We use an element remove-serial-console in image elements.
Renaming this element to match. Also renaming allows us to use
the serial console in devtest runs.
See - https://review.openstack.org/#/c/86272/

Change-Id: I8f1138f58d0046ccdeefa3decc45c36d95c6bf4b
Merge "Explicitly name element enable-serial-console"