Fix exec not working for another region

[legege]

Fix for issue #10538

[hashicorp-cla]

All committers have signed the CLA.

[backspace]

Thanks for the start on this, @legege. I added another commit to address two nuances:

1. localStorage.nomadActiveRegion is effectively a cache for the region most recently chosen from the sidebar dropdown. Changing the region in the sidebar writes to localStorage but service:system.activeRegion is the source of truth for the frontmost application’s active region because localStorage is shared among all windows, but different windows/Ember applications can have a different activeRegion.
2. In a setup without regions, this parameter should not be included in the exec call.

ETA this is not currently possible to exercise within Ember tests; socket URL construction could be extracted for testability, but I didn’t find that too helpful with the construction of exec popup window URLs 😞 but I tested this with both a multi-region setup and a non-region setup.

[DingoEatingFuzz]

LGTM

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