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 entries to pip-requires and pydist-overrides to get it over the hump
where needed.

Fixes Jira: OPENSTACK-760

Change-Id: I11e1eb79edf0208058c95dcefd9187b68736449c