Refactor git_metadata step and yaml config. Add machine_setup and
branch support.

This adds the implementation of the machine setup step.
Implements directory location and branch support for git clone.
Refactors git_metadata step to be test driven.
Adds tests for git_metadata step.
Combines multiple yaml steps into one step for prebuild.

Tested: I ran unit tests with pytest to test for function behaviors
of git_metadata step. All test cases passed.

Signed-off-by: Simon-Lii <thesimonli@google.com>
Change-Id: Ia3524b15313c08140b9a55cbe3d2aa4fbf578744