Use host keytab to obtain credentials needed for ipa-certupdate
This command doesn't require any permissions that a host doesn't
already have and it simplifies overall credential handling.
It also corrects the case where the server API cache is out of
date and there are no credentials available to refresh it which
can lead to a confusing error message.
Also switch to MEMORY-based ccache rather than file to avoid
having to clean up the filesystem.
Signed-off-by: Rob Crittenden <email@example.com>
Reviewed-By: Fraser Tweedale <firstname.lastname@example.org>