Adding support for workspace secret per namespace

[jtyr]

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This PR is adding the possibility to have workspacesecrets per namespace if the TFCO is run to watch all namespaces (when there is --k8s-watch-namespace=<namespace> command line option specified). This allows to have multiple teams, where each team occupies one namespace, to have their own secrets that are passed to Terraform. This PR needs to be rebased once the PR #129 is merged to remove most of the changes in the CRD.

The secret is referenced in the Workspace using the spec.secretName:

...
spec:
  secretName: mysecret
...

The secret referenced by with the spec.secretName must be located in the same namespace like the Workspace resource. Those secrets are merged with the workspace secret in the namespace where TFCO is installed and that is mounted into the container using the spec.secretsMountPath.

Release note for CHANGELOG:

Added support for workspace secret per namespace

[jtyr]

This PR is rebased now.