heat tests : fix run_tests.sh pep checks

run_tests.sh parses tox.ini, which has been changed removing the
pep8 version, so remove this test and specify the version in
test-requires.  This allows the pep tests to run via run_tests.sh
again, a subsequent patch can reinstate the version check based
on the test-requires version

fixes bug 1131251

Change-Id: Ib24f588cbe81ded768376c138d9d89f5bb311185