azure/auth: fix config path parsing

[fairclothjm]

Closes #1186

[raymonstah]

should we add a test case for an invalid path too?

[fairclothjm]

@raymonstah

should we add a test case for an invalid path too?

Not a bad idea, but I couldn't come up with a good way to test that. I am open to suggestions if anyone has any :-)

[raymonstah]

I was thinking of a unit test for the azureAuthBackendConfigBackendFromPath function:

func TestAzureAuthBackendConfigBackendFromPath(t *testing.T) {
	tcs := map[string]struct {
		path        string
		response    string
		expectedErr error
	}{
		"invalid-path": {
			path:        "foo/bar/baz",
			response:    "",
			expectedErr: fmt.Errorf("no backend found"),
		},
	}

	for name, tc := range tcs {
		t.Run(name, func(t *testing.T) {
			response, err := azureAuthBackendConfigBackendFromPath(tc.path)
			assert.Equal(t, tc.expectedErr, err)
			assert.Equal(t, tc.response, response)
		})
	}
}

[fairclothjm]

@raymonstah Sure I can do that but in that case I think we should do some refactoring so that other engines can use this regex. Nothing stopping them now since almost everything is in the vault pkg but it would make more sense to move this to a common place and rename it. I will do that when I can find some time.