server status RPC method: deal with servers without a install server

Autotest servers without a installation server are actually a valid
deployment scenario. So, in those cases, do not set the server's
"concern" status.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #740 from clebergnu/server_status_install_server

server status RPC method: deal with servers without a install server