object-storage: use tox for unit tests; fix em too

Add the ability to use tox for unit tests, since it helps us solve the
problem of supporting multiple branches that require different
versions of dependencies, and allows us to possibly support multiple
versions of python in the future.

Also fix the code to work with pre-grizzly environments, by not
requiring the constraints backport.

Also fixed the xattr support to work with both pyxattr and xattr
modules.

And fixed the ring tests to also work without a live /etc/swift
directory.

BUG: 948657 (https://bugzilla.redhat.com/show_bug.cgi?id=948657)
Change-Id: I2be79c8ef8916bb6552ef957094f9186a963a068
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Reviewed-on: http://review.gluster.org/4781
Reviewed-by: Alex Wheeler <wheelear@gmail.com>
Tested-by: Alex Wheeler <wheelear@gmail.com>
Reviewed-by: Anand Avati <avati@redhat.com>