Fix Get-AzureRmResourcePovider with locations.

[TianoMS]

1. Get a full list of locations returned by /providers, and combine it with the ones by /locations. Use the full list to validate provided location.
2. If the provided location is not in this list, we write an error.
3. User can specify Get-AzureRmResourceProvider -Location "global".

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[TianoMS]

On demand job: http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-demand/1024/