script/ptl-tool.py: Label Validation

script/ptl-tool.py: Label Validation

Validated the label and verified the existence of the label at the beginning of build_branch().
This will avoid:
* Assigning invalid testing labels like 'abcd' and other existing labels like 'bug', 'cleanup' etc; as testing labels
* Stops spamming the Label list in the Ceph repo by creating new (invalid format) labels everytime. The labels can be created manually in github and ptl-tool only uses them.

Signed-off-by: Jos Collin <jcollin@redhat.com>