Fix pep8 errors
plankton: Fix the format of image location meta

Fix the format of the image's location metadata. This metadata contains
the account, container and name of an object in Pithos. Since, it's
value is in the form of a URN, the account, container and name of the
object must be quoted. This commit updates the 'split_url' and
'create_url' functions to unquote the values when parsing a location
metadata, and to quote these values before constructing a location
metadata.

Update section in the image API docs for the format of image's metadata.