add ability to skip neutron agents

Currently, there is no option within openstack-service-checks to skip individual neutron agents.

The only option, check-neutron-agents, only allows a toggle for checking all of them. It would be helpful to add a skip_neutron_agent setting, with a list of agents that can be ignroed. This would be helpful in scenarios where there's a known issue with a single machine, and shouldn't mask detecting issues with the rest of a given cloud.