Use git-lfs for remote locking of ensembles during deploy.

[aszs]

Currently when you deploy, a lock is held on the ensemble being deployed but it is only local to the machine that is running unfurl (YamlManifest.lock()). To prevent deploy job from running on another machine at the same time we can use git-lfs to provide remote locking. We just need to call git lfs lock <ensemble>.remotelock and later git lfs unlock <ensemble>.remotelock (I've already added .remotelock lockable to the .gitattributes file we create) -- see https://github.com/git-lfs/git-lfs/wiki/File-Locking for more info. This functionality should be implemented in repo.py

Ideally the .unfurl-home ensemble template would include a package artifact (depends on #93) that installs git-lfs to make sure the git-lfs extension is installed locally -- it seems that all the major distributions, include brew, include it.