res_corosync: Add check for config file before calling corosync apis

On some systems, res_corosync isn't compatible with the installed version of
corosync so corosync_cfg_initialize fails, load_module returns LOAD_FAILURE,
and Asterisk terminates.  The work around has been to remember to add
res_corosync as a noload in modules.conf.  A better solution though is to have
res_corosync check for its config file before attempting to call corosync apis
and return LOAD_DECLINE if there's no config file.  This lets Asterisk loading
continue.

If you have a res_corosync.conf file and res_corosync fails, you get the same
behavior as today and the fatal error tells you something is wrong with the
install.

ASTERISK-24998

Change-Id: Iaf94a9431a4922ec4ec994003f02135acfdd3889