rhbz1120016 - Allow single OpenId authentication.

Offer a configuration setup that allows for just a single open id provider to be configured.
To test this, the standalone.xml security configuration should look like this (for fedora auth):
```xml
<security-domain name="zanata.openid">
     <authentication>
           <login-module code="org.zanata.security.OpenIdLoginModule" flag="required">
                 <module-option name="providerURL" value="http://id.fedoraproject.org/"/>
           </login-module>
      </authentication>
</security-domain>
```
This is already being cached by JBoss' implementation of the `Configuration` class. See `org.jboss.security.auth.login.XMLLoginConfigImpl` for more details.