Updated development.environment.rst

Enhanced the discussion of tools/install_venv.py to say that it
installs the items listed in test-requirements.txt as well as
those listed in requirements.txt.

Re-wrapped the relevant paragraph.

Since the change to delete run_tests.sh and tools/install_venv.py
(https://review.openstack.org/#/c/76627/)
has been abandoned, the doc of install_venv.py should still be
accurate.

Change-Id: I5cfd9f179f40b2418129067d0940bfdbf47512cb
Closes-Bug: #1266671
Merge "Updated development.environment.rst"