Add ability to follow symlinks to DiskStorageBroker

[tjelvar-olsson]

Sometimes people have existing structures where the data has been "renamed" using symlinks to make the data more understandable. In these cases dtool freeze does not pick up the data in these symlinked directories as os.walk will not walk down into symbolic links that resolve to directories.

In this instance it would be useful to be able to change the default behaviour to follow the symlinks. For example by setting an environment variable, e.g.

export DTOOL_DISKSTORAGEBROKER_FOLLOW_SYMLINKS=1