app_confbridge: Default the template option to a compatible default profile.

Confbridge dynamic profiles did not have a default profile unless you
explicitly used Set(CONFBRIDGE(bridge,template)=default_bridge). If a
template was not set prior to the bridge being created then some
options were left with no default values set. This patch makes it so
the default templates are set to the default bridge and user profiles.

ASTERISK-24749 #close
Reported by: philippebolduc

Change-Id: I1bd6e94b38701ac2112d842db68de63d46f60e0a