framework: add SSH support for Azure

Add Azure to the list of providers that support accessing nodes
using SSH.
Note: This will require a follow up PR adding the required
environment variables, AZURE_SSH_KEY, KUBE_SSH_BASTION to the test
configuration.

test/e2e/framework/ssh/ssh.go

@@ -86,6 +86,11 @@ func GetSigner(provider string) (ssh.Signer, error) {
 		if keyfile == "" {
 			keyfile = "id_rsa"
 		}
+	case "azure":
+		keyfile = os.Getenv("AZURE_SSH_KEY")
+		if keyfile == "" {
+			keyfile = "id_rsa"
+		}
 	default:
 		return nil, fmt.Errorf("GetSigner(...) not implemented for %s", provider)
 	}