sip_to_pjsip: Add cert_file.

When using the migration script sip_to_pjsip.py, cert_file was not migrated to
pjsip.conf. A previous change regarding this contained a copy/paste error.

ASTERISK-22374

Change-Id: I0fa72e9412117d53b4284fc6b83fa5b2b95ba03b