Add mojo/tools/mopy with paths and version utilities

This consolidates some common mojo python script utilities into
mojo/tools/mopy and updates scripts under mojo/tools to use them.
Currently this has some path manipulation, version querying and python
test running utilities but I expect this to grow over time. A skypy
could also resuse a lot of of this functionality as well.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/707893005