Make pmd-cpu-set mandatory when enable-dpdk is true

[Giuseppe Petralia]

If pmd-cpu-set is unset, OVS DPDK will still pin the PMD threads to two cores, one per NUMA node,
that for some reasons in different envs always resulted in cpus 11 and 21

The charm should avoid this silent configuration which may lead to not properly configured systems.

i.e.
The operator may ignore that OVS will still pin the pmd threads to certain cores and may not be aware that pmd threads need to be removed from cpu-affinity range and/or nova cpu-{shared|dedicated}-set.

It would be safer, if the charm always requires that the operator explicitly sets the pmd-cpu-set when enable-dpdk is true, to force the operator thinking how cpus are assigned to the different components (OS, hypervisor, OVS DPDK).

[Andrea Ieri (aieri)]

This could cause issues during charm upgrades by setting the charm to a blocked state in all deployments where enable-dpdk=true and pmd-cpu-set is unset, even if that combination is not causing problems. A more desirable approach would be to move away from the pmd-cpu-set configuration altogether, by having the ovn-chassis charm be able to introspect which CPUs can be used.