This makes it faster to use Docker images to test new changes, as
suggested in #492.
Since we don't need to re-bootstrap for every push to master, this also
makes our automated builds faster and more reliable (they often fail due
to network lag when installing dependencies with apt-get).
Lastly, this will make it easier to swap out bootstrap images built for
different flavors, such as MariaDB 10.0 vs MySQL 5.6.
for building. Also properly parsing the command line flag
in bootstrap.sh.
Should reduce te nuber of times we build proto from 4x to 1x, and
grpc from 2x to 1x. Doh.
Anthony Yeh
ZhiFeng Hu
Alain Jobart