<broker> improve Gemfile loading + bug 993250

https://bugzilla.redhat.com/show_bug.cgi?id=993250
Bundler load failures are wrapped and re-interpreted for OpenShift
  instead of telling the user to "bundle install"
OPENSHIFT_CONF_DIR overrides /etc/openshift base for broker config files
Gemfile loads plugins even if just the -dev.conf file is present
Plugins are loaded in sorted order, in case it matters
Merge pull request #3294 from sosiouxme/broker-plugin-loading

Merged by openshift-bot