It's test.py --check-deletions

Rename the option as suggested by Anne in
https://review.openstack.org/48128
Remove now obsolete options --gate-build and --gate-niceness.
Add option check-all.

Change-Id: Id918bf22a33a485b2bd398292d3ae58b4d39a535
Merge "It's test.py --check-deletions"