login concurrency failure due to use of az login

[blueskyjunkie]

Describe the bug
Concurrency in the deployment engine intermittently results in deployment failures - typically with an error along the lines of "subscription not found" on attempted use of az login or az account set.

az login uses a single file to record the login state, so it can only be logged in to a single subscription at a time. In addition, concurrent application steps may change the login file resulting from a new login attempt, even to the same subscription and corresponding changes using az account set, that causes failure of a different application step trying to consume the login (hence "subscription not found"). Essentially, az login does not support any kind of concurrency with respect to logins.

To Reproduce
It's intermittent so not generally reproducible, although deployments with high levels of concurrency are more likely to exhibit the issue.

Expected behavior
The deployment should be able to concurrently deploy to any number of subscriptions simultaneously.