Dockerfile: Optimize pip installs
Dockerfile: Optimize pip installs

With this change, we can avoid rerunning bootstrap on build if
requirements.txt hasn't changed. Build times after non-requirements
changes go from 3-5min to ~2s.

Signed-off-by: Zack Cerza <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@redhat.com>

ktdreyer/teuthology,ktdreyer/teuthology,ceph/teuthology,ceph/teuthology