Add a default hawtio.roles setting to etc/system.properties

Currently hawtio's hard-coded to accept 'admin' by default, we should however configure hawtio to allow the pre-defined set of roles, I would suggest for the Fuse/AMQ distro in etc/system.properties:

hawtio.roles=admin,manager,viewer,Operator,Maintainer,Deployer,Auditor,Administrator,SuperUser,User

Would be good to get this into the hawtio profile so the setting is consistent in a fabric as well.