Drop redundant Depends: lines from debian/control

dh_python2 figures all this out, and apparently does a better job
at it than we do.  Switch to just using ${python:Depends} and clean
up continuation lines where helpful.

Add pydist-overrides entries with PEP386 info to help it get over the
hump where needed.

Fixes Jira: OPENSTACK-760
Change-Id: I5737c1a415269c4656ab0fecf7cf735d4fbf1cc4