Python merge use git choosing strategy default

Python merge use git choosing strategy default

Use git choosing strategy default in tox and pypi merge jobs for
gerrit.  This makes those jobs consistent with maven and other
merge jobs for gerrit that always build from tip of the target
branch to create artifacts from the latest & greatest code.
Building from tip (not from Gerrit commit/merge point) avoids
confusion about content when changes are merged out of order.
For example, a fix is submitted and merged, but the merge job
fails. In the mean time, a different change that happened
earlier in commit history gets merged (or the merge job is
retriggered), causing a new artifact to be pushed.  But that
artifact does not have the expected fix.

Add comments to release merge jobs why their choosing strategy
is not default.

Document the git commit choosing strategy for the release merge jobs.

Change-Id: I2e32dfa946d678fa59a290ddb53fc09387d59baf
Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>