res_pjsip_pubsub: Treat "prune_on_boot" as a yes / no.

The alembic for the PJSIP subscription persistence table has the
"prune_on_boot" field as a boolean. While in Asterisk we are
tolerant of many different definitions of true and false in the
database we only accept "yes" and "no". This change makes the
field treated as a yes/no instead of an integer, thus storing
"yes" and "no" instead of "1" and "0".

Change-Id: Ic8b9211b36babefe78f70def6828a135a6ae7ab6