Ensure namespace is set on all Kubernetes resources

[RyanW8]

What this PR does:

Ensure the namespace defined in _config is set on these resources:

• Distributor
• Memcached
• Querier
• Query-frontend Which issue(s) this PR fixes: Fixes #177

Checklist

• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[RyanW8]

Fixed conflicts. Can you please merge?

[pstibrany]

Branch https://github.com/grafana/cortex-jsonnet/tree/pull/178 shows how to add namespaces to more (but still not all) resources.

[CLAassistant]

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:x: Ryan
:x: RyanW8

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