http.c: Reload TLS even if http.conf hasn't changed

There is currently no way to indicate to Asterisk that TLS certificates
and/or keys have been updated other than by modifying http.conf or
restarting Asterisk.

There is already code in main/tcptls.c that determines if a reload is
actually necessary based on the hashes of the certicate and dependent
files, so this change merely gives us a way to request a reload without
explicitly modifying http.conf.

Change-Id: Ie795420dcc7eb3d91336820688a29adbcc321276