Allow to use GetSigner with vagrant provider

In order to run tests that require ssh access to a node on vagrant
we need to provide path to private ssh key.

Now it will be possible to do using VAGRANT_SSH_KEY environment variable

Change-Id: Ic5fe0037edd46d0db3b8036ad7fc03cf1ea07574

test/e2e/framework/util.go

@@ -4098,6 +4098,12 @@ func GetSigner(provider string) (ssh.Signer, error) {
 		}
 		// Otherwise revert to home dir
 		keyfile = "kube_aws_rsa"
+	case "vagrant":
+		keyfile := os.Getenv("VAGRANT_SSH_KEY")
+		if len(keyfile) != 0 {
+			return sshutil.MakePrivateKeySignerFromFile(keyfile)
+		}
+		return nil, fmt.Errorf("VAGRANT_SSH_KEY env variable should be provided")
 	default:
 		return nil, fmt.Errorf("GetSigner(...) not implemented for %s", provider)
 	}