Add tempest-lib as a global requirement and a tracked project

This commit adds tempest-lib to global requirements and also adds it
to project.txt. This will only be a test-requirement on projects,
except for tempest.

This is needed so tempest can migrate to using tempest-lib. The stable
jobs run with tempest master, however since tempest-lib isn't in
requirements for the stable branches these jobs can't pass which blocks
the tempest changes from landing.

Change-Id: Iaf6228a5e0968e9578abd93d2d0e47cf6a089c75
(cherry picked from commit ed1126717eedfed036888aa56f4fc6000b4fc6a6)