label_selector parser is too draconian re white-space

See also https://github.com/cloudfoundry/cloud_controller_ng/issues/1443

Summing up 5 known issues:

Given this label_selector: environment in (production, staging), tier=backend

There are at least 4 known errors related to white-space handling:

1. Space required before open-paren:

environment in(production, staging), tier=backend
                        ^

2. Space forbidden inside comma-separated in-block (issue #1443):

environment in (production, staging), tier=backend
                                             ^

3. Space forbidden after and-type comma:

environment in (production, staging), tier=backend
                                                             ^

4. Spaces forbidden around equal-sign:

environment in (production, staging), tier =backend
                                                                   ^

5. Trailing spaces forbidden

environment in (production, staging), tier=backend[SPACE]
                                                                                   ^

6. Leading spaces forbidden

[SPACE]environment in (production, staging), tier=backend
^

The examples in the Kub document appear more forgiving of white-space. They don't talk about it, at least

Locate the label_selector parser and compare its whitespace rules with the Kubernetes document referenced in the issue. Reproduce the six examples, then verify that leading, trailing, separator, parenthesis, and equals-sign whitespace are handled as intended.