installer: rename --subject to --subject-base
The --subject option is actually used to provide the "subject base".
We are also going to add an option for fully specifying the IPA CA
subject DN in a subsequent commit. So to avoid confusion, rename
--subject to --subject-base, retaining --subject as a deprecated
alias.
Part of: https://fedorahosted.org/freeipa/ticket/2614
Reviewed-By: Jan Cholasta <jcholast@redhat.com>