Consolidate Python Label Flag Handling

Python GoogleCloudOptions \--labels flag ([https://github.com/apache/beam/blob/master/sdks/python/apache_beam/options/pipeline_options.py#L371](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/options/pipeline_options.py#L371)) ) diverges from the Java and Go implementations and should have its functionality consolidated. Java and Go expect \--labels=; python allows passing multiple \--label x=y (or \--labels x=y) flags.

Given that this flag has already been included in a released Python SDK, fixing this bug either requires addition of a new flag or detection of format for existing flag. This decision is left to the discretion of the implementer.

Imported from Jira [BEAM-4243](https://issues.apache.org/jira/browse/BEAM-4243). Original Jira may contain additional context.
Reported by: jaku.