Azure offers different endpoints to access vault

[sebagomez]

Right now the url to access the vault is concatenated with a fixed string template: fmt.Sprintf("https://%s.vault.azure.net", a.keyvault)

That url is actually different depending on the Access plane (Global, Azure China 21Vianet, Azure US Government & Azure Germany).

There should be a way of configuring that url instead of having the fixed one hard coded.