Allow for specifying IP pool configurations via charm settings

[Paul Goins]

While not critical (as this can be easily accomplished post-deploy via a few calicoctl commands), it would be nice if there was an option to pre-define the desired IP pools via the charm config. This might be as simple as allowing for specifying a multi-document YAML blob defining the pools.

Use case: This would be helpful in the case that there's a need for different routing rules depending on the type of workload; multiple IP pools could be specified and policy routing could use these CIDRs to route appropriately.

Post-deploy procedure today:
* calicoctl delete pool default
* calicoctl apply -f <new_pools_yaml>
* It might be necessary to restart pods started with incorrect IPs; I'm not sure though.