Fix #git_co error message and rename internal variables
Add a utility method to return the git sha of HEAD

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Add sha to variables tracked within build

This allows us to check out a specific sha prior to build, in case the source
repository has changed since the build was submitted to the remote build
system.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>