certdb: accumulate extracted certs as list of PEMs
certdb.NSSDatabase.import_files currently accumulates certificates
extracted from input files as a string, which is ugly. Accumulate a
list of PEMs instead, and join() them just in time for PKCS #12
creation.
Part of: https://fedorahosted.org/freeipa/ticket/6178
Reviewed-By: Jan Cholasta <jcholast@redhat.com>
Reviewed-By: Tomas Krizek <tkrizek@redhat.com>