Refactor machine_setup step and add gsutil_setup step.

This refactors machine_setup to be test driven.
Adds the gsutil_setup step that creates the sstate-cache and the
sources mirror URL directories.
Adds support to decide between sstate-cache or clean build.
Adds unit tests for machine_setup and gsutil_setup.

Tested: I ran unit tests with pytest to test for function behaviors
of all the prebuild classes. All test cases passed. Performed manual
end-to-end tests on GCB with sstate-cache, all test cases passed.

Signed-off-by: Simon-Lii <thesimonli@google.com>
Change-Id: I392e100119e002af6b2c49a9e074066eb1bbfe5b