USHIFT-601 create_pr.py: get base branch from job's name

[pmtk]

Based on job environment variables:

...
JOB_NAME=periodic-ci-openshift-microshift-main-periodics-rebase-on-nightlies
...

Tested in python's repl:

>>> import re
>>> re.search("periodic-ci-openshift-microshift-(.+)-periodics-rebase-on-nightlies", "periodic-ci-openshift-microshift-main-periodics-rebase-on-nightlies").group(1)
'main'

>>> re.search("periodic-ci-openshift-microshift-(.+)-periodics-rebase-on-nightlies", "periodic-ci-openshift-microshift-release-4.12-periodics-rebase-on-nightlies").group(1)
'release-4.12'

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pmtk

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[pmtk]

/test verify

[openshift-ci]

@pmtk: all tests passed!

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[pmtk]

/hold

[openshift-merge-robot]

@pmtk: PR needs rebase.

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[pmtk]

Obsoleted by #1242 and #1245