Run CI on CentOS 7

[andrewsykim]

A lot of Kubernetes users (using IPVS proxy) run on CentOS 7 which is based on Linux 3.10. In the past there have been compatibility issues with older kernel where certain netlink attributes do not exist. For example, for Linux < 3.18 the address family doesn't exist so we had to create some workarounds (#15 and #19). The address family compatibility issue would have been caught if we ran tests on CentOS 7.

[andrewsykim]

Looks like we would need to use self-hosted runners for this actions/runner-images#45

[cpuguy83]

Yes, for any specific targets we'd need self-hosted runners, otherwise we'll get Azure kernels.