Add cert checks in ipa-server-certinstall
When ipa-server-certinstall is called to install a new server certificate,
the prerequisite is that the certificate issuer must be already known by IPA.
This fix adds new checks to make sure that the tool exits before
modifying the target NSS database if it is not the case.
The fix consists in creating a temp NSS database with the CA certs from the
target NSS database + the new server cert and checking the new server cert
validity.
https://fedorahosted.org/freeipa/ticket/6263
Reviewed-By: Jan Cholasta <jcholast@redhat.com>