## Motivation
Testing #4167 has revealed some `linkerd check` failures that occur only
because the checks happen too quickly after cluster creation or install. If
retried, they pass on the second time.
Some checkers already handle this with the `retryDeadline` field. If a checker
does not set this field, there is no retry.
## Solution
Add retries to the `l5d-existence-replicasets`
`l5d-existence-unschedulable-pods` checks so that these checks do not fail
during a chained cluster creation > install > check process.