* azuremonitor: revert to clearing chained dropdowns

After feedback from users, changing back to clearing
dropdowns to the right in the chain. E.g. if the user
changes the subscription dropdown which is first in
the chain then all the dependent dropdowns to the right
should be cleared (reset to default values).

Also, now triggers getting subscriptions every time the
dropdown menu is shown rather than just the first time.
It is apparently common to add subscriptions while
building queries.