Only lookup secrets in the namespace where the install is targeting.

[t-eckert]

Changes proposed in this PR:

• The install command will only look for secrets in the namespace the install is targeting.
  This fixes the issue raised by @lkysow.

  another one, it's complaining I have a secret in my default ns but I'm installing consul into the consul ns:

  consul-k8s install
  ==> Checking if Consul can be installed
  ✓ No existing Consul installations found.
  ✓ No existing Consul persistent volume claims found
  ! Found Consul secrets, possibly from a previous installation.
  Delete existing Consul secrets from Kubernetes:
  
  kubectl delete secret consul-federation --namespace default

How I've tested this PR:

Extended existing unit tests to cover this case.

How I expect reviewers to test this PR:

👀 and running unit tests.

Checklist:

• Tests added
• CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry.
  Entries should use present tense (e.g. Add support for...)

[jmurret]

Nice!