Add git lfs to lambda layer

[freakingawesome]

Resolves lambci/git-lambda-layer#22

This adds the appropriate git-lfs RPM repo and validates its signature, then installs it with the yum install command. We have to manually move the git-lfs binary (which is the only file required for git-lfs) into the /opt/bin/ folder of the output so that it will be available in the $PATH in the lambda layer.

This manual handling of the RPM repo is done because the standard git-lfs install script has hardcoded /etc/ all over the place, whereas we need it to respect the /lambda/ root.

[brondibur]

@mhart @freakingawesome Hi guys, any update on when this feature will be added?