ipa-ca-install: do not fail without --subject-base and --ca-subject
When --subject-base and --ca-subject are not specified in ipa-ca-install,
default values are used. DN objects are used as the default values in
ipa-ca-install, but the CA installer expects the values to be strings. This
causes ipa-ca-install to fail unless both --subject-base and --ca-subject
are specified.
Convert the DN objects to strings to fix the issue.
https://fedorahosted.org/freeipa/ticket/2614
Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>