[neutron] various improvements to check_neutron-server.sh

    - add -m script option to define AMQP port instead of grepping an hardcoded amqp string
    - add -n option for netstat not to resolve hostnames and ports
    - fix bug if keystone does not send a token
    - fix bug if keystone is not reponding
    - fix bug if neutron API is not responding
    - fix bug in PID matching in netstat output
    - enhance user-provided arguments (script now exits if no credentials are given)
    - more bash compliant
    - script now outputs a nagios state string at each exit
    - make error messages more verbose