Location completer sometimes not working

## Description

Location completer returns no suggestions.


## Root cause

When user hits tab, `LocationCompleter` start sending a request to list all resource providers, and it usually takes ~3.5 seconds (on my machine) to finish


But our code has set it to timeout in 3 seconds.

https://github.com/Azure/azure-powershell-common/blob/master/src/ResourceManager/Version2016_09_01/ArgumentCompleters/LocationCompleter.cs#L37

So `LocationCompleter` doesn't get the data and it cannot do any completions.

## Possible solutions

* Figure out why it's taking so long to list providers
* Increase timeout limit
* easiest solution but it could make tab-completion slower
* Continue the request in the background even after it timeouts (so user can get completion next time)
* (By Dwayne Need) if the auto-complete fails, don’t let the default auto-complete kick in and fill in some nonsense file in my local directory.

## Cost
?