Refactor away use of `az` CLI

[greggroth]

The az bash CLI is an implicit requirement to get the master key here:

cosmosdb-powershell/cosmos-db/cosmos-db.psm1

Lines 70 to 75 in d32805b

	if ($SubscriptionId) {
	$masterKey = az cosmosdb keys list --name $Database --query primaryMasterKey --output tsv --resource-group $ResourceGroup --subscription $SubscriptionId
	}
	else {
	$masterKey = az cosmosdb keys list --name $Database --query primaryMasterKey --output tsv --resource-group $ResourceGroup
	}

it would be better if this was refactored to use Get-AzCosmosDBAccountKey so the bash CLI isn't required