Doc changes for new pip-requirements

dnspython was added recently to the list of requirments.

This patch completes the change:
1. The new requirement is documented
2. The need for tox --recreate is documented
(assuming there is no way to automate 2 above)

Change-Id: I49f0cb4b01b79dd7cf4329984a2f8416378ebe19
Fixes: Bug #1176627
Merge "Doc changes for new pip-requirements"