ca-less tests: fix getting cert in pem format from nssdb
usage of ipautil.run in get_pem methond of ca-less tests was not
refactored when the ipautil.run was refactored in
099cf98307d4b2f0ace5d5e28754f264808bf59d
This results in failure of all CA-less test.
https://fedorahosted.org/freeipa/ticket/6177
Reviewed-By: Martin Basti <mbasti@redhat.com>