Resolve Issues SONARPLUGINS-2931 (Update pom.xml) and SONARPLUGINS-3028 (Use the sonar.security.realm property)

This fix is to change CrowdPlugin to use the new CrowdRealm, which extends SecurityRealm, instead of CrowdAuthenticator.
As a result, one needs to change the SONARQUBE_HOME/conf/sonar.properties file accordingly:

sonar.authenticator.class: org.sonar.plugins.crowd.CrowdAuthenticator
to
sonar.security.realm: Crowd

This version is not backwards compatible with the old configuration using sonar.authenticator.class.