Image server should not wipe imported images by default, nor its publish_path

[Caio Begotti]

Currently the image server code wipes out all images after they have been processed from copy-images or import-images, which IMHO is plainly wrong. Removing files should be optional, not default, as image server may be sitting next to a storage or archiver of phone images (like we do on Prodstack).

Also, by doing so it wipes out its own publish_path too, so it may render next runs of import-images broken:

Traceback (most recent call last):
  File "bin/import-images", line 66, in <module>
    pub = tree.Tree(conf)
  File "/tmp/system-image/bin/../lib/systemimage/tree.py", line 153, in __init__
    raise Exception("Invalid path: %s" % path)

[Caio Begotti (caio1982)]

The related branch solves this problems for us while also keeps the original behavior available through a command line option in both copy and import images scripts.