[SONAR-4983] refactored security realms

Hi

I implemented [SONAR-4983] (https://jira.codehaus.org/browse/SONAR-4983) and would be glad if someone of you could have a look at this change and merge it if you think it is fine.

I tested it together with [sonar-preauth] (https://github.com/stadler/sonar-preauth) and [sonar-ldap] (https://github.com/SonarCommunity/sonar-ldap) and it worked as expeced.

This change basically allows you to use different authenticators and user- and group-providers together. 
In my setup for example, I use preauth for authentication and ldap for the user and group details. The different authenticaors and user- and group providers can be defined in the sonar properties by three new properties: 
- `sonar.security.authenticators`: the class names of the classes that will be doing the authentication. E.g. org.sonar.plugins.preauth.PreAuthAuthenticator
- `sonar.security.userProviders`: the class names of the classes that will do the provisioning of the user details. E.g. org.sonar.plugins.ldap.LdapUsersProvider
- `sonar.security.groupProviders`: the class names of the classes that will do the provisioning of the group details. E.g. org.sonar.plugins.ldap.LdapGroupsProvider

Note that if a security realm is defined by using `sonar.security.realm`, then the defined security realm takes precedence.

Thank you for having a look at it and don't hesitate to comment if you have any questions.
   Jacques