Extracting stage releasenotes script from stage-test.sh

Release notes checking in stage-test.sh causes the failure in  merge and revert jobs.
So it is added as an separate script.

Change-Id: Ie20878cb4822ba72e9d81f3e6ede8dcd1f65d99e
Signed-off-by: aticig <gulsum.atici@canonical.com>