Fix `SproxydException.__str__`

The original implementation had a side-effect modifying an attribute of the
exception instance, which resulted in unexpected behaviour when rendering some
instance multiple times.
Harden `sproxyd_host` setting parsing

This changes allows putting some whitespace in-between addresses in the
configuration file, e.g.::

    sproxyd_hosts = 192.168.1.1, 192.168.2.1
Cycle through hosts instead of picking a random one
Add proper `__repr__` to `SproxydException`
Minor `pylint`-directed cleanups
Revert "Remove shallow conn_getresponse() method"

The original change breaks all test-cases, so this can only be applied once we
figure out how this impacts the tests & how to work around the change.

This reverts commit 8d9c3739df3aafe64b151e0185342962cf548d18.

Conflicts:
	swift/obj/scality_sproxyd_diskfile.py
Merge pull request #4 from scality/fixups

Various fixups