dhcp: make netns path absolute prior to RPC
dhcp: make netns path absolute prior to RPC

The dhcp daemon may be running with a different cwd
and so the netns paths need to be absolute. This patch
also refactors the code to pull out the common, RPC
parts, out into a separate function.

squaremo/cni,achanda/cni,zenlinTechnofreak/cni,containernetworking/plugins,mcastelino/cni,Symmetric/cni,squaremo/cni,appc/cni,containernetworking/cni,dcbw/cni,steveeJ/cni,luxas/cni,steveeJ/cni,murat1985/cni,rosenhouse/cni,appc/cni,tomdee/cni,NeilW/cni,tomdee/cni,kad/cni,containernetworking/cni,squeed/plugins,kad/cni,zenlinTechnofreak/cni,Symmetric/cni,jonboulle/cni,jonboulle/cni,tangle329/cni,luxas/cni,murat1985/cni,containernetworking/plugins,aanm/cni,rosenhouse/cni,mcastelino/cni,tangle329/cni,dcbw/cni,squeed/plugins,achanda/cni,aanm/cni,NeilW/cni