Support bootstrapping an environment into a namespace

[laszlocph]

Today you have to dedicate a k8s cluster for an environment.

This issue is about to make possible to use a namespace for an environment.

Notes:

• agent name should contain the env name
• env should have an staging=staging syntax
• One click agent config for namespace binding
• gimletd should know about the ns restriction
• dash should prefill ns, prevent edit
• in bootstrap, don't install flux