acl: don't crash if relay_dest_domains.ini missing

plugin attempts to access an section of this config file. If the config file is missing, it crashes. This checks for the required section and skips domain test if missing.

Also updated the log entry and variable names to reflect this plugins current name.
acl: added a test for relay_dest_domain
Merge pull request #499 from msimerson/acl

acl: don't crash if relay_dest_domains.ini missing