add "scope" parameter

[Alvaro Uria]

This is a proposal for a new parameter:
scope={general,relation,all}
(default: all)

When deploying OpenStack on baremetal, several services can converge into the same nodes:
* os-controllers, os-storage, os-compute: being generic ubuntu charms to identify roles and hardware specs
* nova-compute, ceph-osd, swift-storage-z1: being openstack charms with specific nrpe checks shared via subordinate relation

Ideally, I'd want generic services (os-controllers, ...) to handle "scope: general" checks (swap, load, disk usage), via a ie. nrpe-physical-controller service

OTOH, openstack charms could relate to nrpe-physical-services, which would handle "scope: relation" (checks defined on the related charms). This would also apply on services running on containers, where we don't want duplicate checks (ie. swap usage).

Finally, "scope: all" would make the charm handle both general and specific (related) checks, as it happens now. This should be the default value for the new "scope" parameter.

[Alvaro Uria (aluria)]

I think this feature is not required. Any generic check (load, swap, disk usage) can be disable by setting its value to '' (empty string).

The scenarios we run in BootStack are:
* scope: all -> generic checks are configured, as well as the logic enabled by relations (ie. check swift replication lag)
* scope: relation -> only enable checks related to relations (avoid generic ones, since another principal unit running on the same node may already have configured them).