Add gitfs "mountpoint" feature, and per-remote root/mountpoint
Add default value for gitfs_mountpoint
Add gitfs_mountpoint master conf parameter to docs
Make file.recurse work with gitfs_mountpoint

When a file.recurse state is specified for salt://foo, and there is a
gitfs_remote mounted at salt://foo/bar, when the fileserver is queried
for the list of directories on the master, 'foo' will not be among them,
causing a pre-flight check to fail and keeping the state from running.
This commit modifies that pre-flight check to also look for paths that
start with 'foo/' as well, allowing salt://foo to be properly matched
when a mountpoint is used.