In certain scenarios, the service-mirror may act as a single point of
failure. Linkerd's multicluster extension supports an `--ha` mode to
increase reliability by adding more replicas, however, it is currently
supported only in the gateway.

To avoid the service-mirror as a single point of failure, this change
introduces an `--ha` flag for `linkerd multicluster link`. The HA flag
will use a set of value overrides that will:

* Configure the service-mirror with affinity and PDB policies to ensure replicas
  are spread across hosts to protect against (in)voluntary disruptions;
* Configure the service-mirror to run with more than 3 replicas;
* Configure the service-mirror deployment's rolling strategy to ensure
  at least one replica is available.

Additionally, with the introduction of leader election, `linkerd mc
gateways` displays redundant information since metrics are collected
from each pod. This change adds a small lookup table of currently lease
claimants. Metrics are extracted only for claimants.

Signed-off-by: Matei David <[email protected]>

…-mirror-ha

Signed-off-by: Matei David <[email protected]>

Signed-off-by: Matei David <[email protected]>

Signed-off-by: Matei David <[email protected]>

… into matei/service-mirror-ha

Signed-off-by: Matei David <[email protected]>