Reuse process management classes from dnsmasq for radvd

[Henry Gessau]

When reviewing/discussing the change to add functional tests for radvd[1] it was requested that radvd should be managed similar to the dnsmasq process. We should reuse the classes already existing for dnsmasq. Extract common functionality to allow them to work for both radvd and dnsmasq. This will allow some common functional testing too.

[1] https://review.openstack.org/109889