Fix useradmin dependancy

This change makes it the dependancy on the org.osgi.service.useradmin package completely optional. This means that the osgi.cmpn package is no longer needed to run FPAI, when the UserAdmin is not used.

This prevents package conflicts such as the ones we had with the ConfigurationAdmin from felix (they delivered 1.5 of the org.osgi.service.cm package, the osgi.cmpn delivered the 1.4).