Adds git sha scope for variable injection

[bomoko]

This PR suggests a way of controlling scope for the GIT_SHA variable, as described in #210

Closes #210

[shreddedbacon]

My only concerns with this are that the environment_variables.git_sha field goes from being a boolean, to a string. I think it is ok though.

Edit: And this field isn't in the go rewrite yet either, so I'm more comfortable with it changing now before it gets merged into the newer code base.

[shreddedbacon]

Maybe it makes sense to just set LAGOON_BUILD_GIT_SHA to be the git sha all the time, this way users can consume it if they need to without needing to modify their sha settings when they may only need this value in certain images. as LAGOON_GIT_SHA influences an entire build process.

There is also the LAGOON_BUILD_NAME that can be consumed as a build argument if required for a unique variable in each build that consumed as required by users if they need something that changes reliably during a build.