Add support for custom attributes for services

Automation can now use the "custom_keys", "custom_set", and "custom_get"
methods to manipulate the custom attributes on a service.  Any associated
custom attributes are now visible on the service details page.

https://bugzilla.redhat.com/show_bug.cgi?id=1108331
Merge pull request #13 from dillaman/bz1108331-service-custom-attributes

Add support for custom attributes for services