* Split remote cluster watching from reversetunnel.AgentPool
Separating the responsibilities:
- AgentPool takes a proxy (or LB) endpoint and manages a pool of agents
for it (each agent is a tunnel to a unique proxy process behind the
endpoint)
- RemoteClusterTunnelManager polls the auth server for a list of trusted
clusters and manages a set of AgentPools, one for each trusted cluster
Previously, AgentPool did both of the above.
Also, bundling some cleanup in the area:
- better error when dialing through tunnel and directly both fail
- rename RemoteKubeProxy to LocalKubernetes to better reflect the
meaning
- remove some dead code and simplify config structs
* reversetunnel: factor out track.Key
ClusterName is the same for all Agents in an AgentPool. track.Tracker
needs to only track proxy addresses.
