Add modules to install gitfs backend dependencies

Add modules for gitfs dependencies and documentation for how to use them.

salt/gitfs/gitpython.sls

@@ -0,0 +1,13 @@
+{% from "salt/map.jinja" import salt_settings with context %}
+
+{% if salt_settings.gitfs.gitpython.install_from_source %}
+
+GitPython:
+  pip.installed
+
+{% else %}
+
+python-git:
+  pkg.installed
+
+{% endif %}