Populate devtest_variables from devtest_setup.

Shift great hunks from devtest_setup (all of the things like exporting
variables) into devtest_variables. This allows developers to just
source devtest_variables.sh into their environment to run the other
scripts that depend on the variables existing in the environment.

Change-Id: Idd412616a547c5af1c13ead8bcdc911a26c5527b