Centralize setting of OVS_RUNDIR (and similar variables)

MicroOVN command wrappers use variables like OVS_RUNDIR, OVN_RUNDIR and others. Currently a lot of wrappers set these variables on their own.
It'd be better if there was one, or possibly more, env files that set values for these variables and the wrappers would just source them. There already are files `ovn.env`[0] and `ovn-central`[1] that do this, but we should finalize the effort.

[0]https://github.com/canonical/microovn/blob/7b4df03ab8baffcb42766518f0bd3fe35f6d643b/snapcraft/ovn.env
[1]https://github.com/canonical/microovn/blob/7b4df03ab8baffcb42766518f0bd3fe35f6d643b/snapcraft/ovn-central.env