APM Central Management - Ability to apply configuration to a subset of agents with same service & environment

**Is your feature request related to a problem? Please describe.**

It is possible to have multiple instances of the same service on a given environment.
In some cases we want to change the setting of a subset of such instances via APM Central Management.

**Describe the solution you'd like**

At the moment, the only criterias are `service` and `environment`.

It would be nice to move to a tag-based approach (e.g. treat `service` and `environment` as "base" tags).
On top of those 2, it would be useful to add `host` (or eventually other `container`-derived settings) or `datacenter`, so it would be possible to specify a set of instances/hosts.
This would allow having a lot of flexibility.

E.g. tags available on a given agent: `service:myservice`, `environment:mytest`, `host:myhostname`, `datacenter:xyz`.

**Describe alternatives you've considered**

The workaround at the moment is to create a "fake" environment and change the name of the environment on the applications, which requires an application restart.