When specifying the tags TLK, stacker removes the stacker_namespace tag

This is more of a bother than a real issue. When a stacker config yaml has tags defined, it removes the default stacker_namespace tag. This is not necessarily the expected behavior.

https://github.com/remind101/stacker/blob/ecd8933d715da8ede5b0d6e6d1e98bd827ff6b43/stacker/context.py#L97

When tackling this issue, we have to be cautious of end-users who have workarounds in place to add the stacker_namespace tag back in via the yaml config. Stacker should not crash in this scenario.