Add MANIFEST.in

The python-jenkins 0.3.3 release is missing requirements.txt.
Investigation shows that python-jenkins was recently switched to
using pbr, and has no MANIFEST.in.  This adds a MANIFEST.in,
modeled on the one in use by nova

Change-Id: I49bdd5377abd2d0384598c1e927f2dd7f6dec978
Closes-Bug: #1360348