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
Lock around ubuntu tarball download

Make this safe for parallel builds

Change-Id: I578fa38e1d44bc205b70d087d161b471cd260fa1
Cleanup apt cache after grub install

Grub installation happens in finalize.d so cleanup should be done after this.

Change-Id: I6322b8b529e31fd1b7251dd9e07cc85f442f7e81
Merge "Lock around ubuntu tarball download"
Merge "Explicitly name element enable-serial-console"