sip_to_pjsip: Map the TLS method correctly.

When using the migration script sip_to_pjsip.py and tlsclientmethod is not set
in sip.conf, the default value of chan_sip (sslv23) is copied to pjsip.conf, to
overwrite the default of the PJProject (tlsv1). This makes sure, res_pjsip is
offering/using not just TLSv1.0 but TLSv1.2 as well.

ASTERISK-22374

Change-Id: Ie530a3dae9926ae14f3920a21be1e2edb15bda4f