[stable/ambassador]better control for namespace scope

[ryandawsonuk]

Currently you can set env.AMBASSADOR_SINGLE_NAMESPACE to tell ambassador to only watch resources in the current namespace. But if you have a chain of dependent charts then once one turns this on then you can't turn it off again. This is because ambassador expects the var to either be set or not set and unsetting a value in a map is problematic.

It would be smoother to make this its own variable wrapped in an if statement, as it used to be at some point.

For other env vars it is fine to use the env map but this one is better handled by wrapping in an if statement as ambassador works by looking at whether it is set or not set, not by looking at the value of the var