Move flake8 option from run_tests.sh to tox.ini

flake8 will read the section [flake8] of tox.ini to guide the behavior.
In run_tests.sh, there is no need to add flake8 option in run_flake8()
function.

Bug #1214175

Change-Id: If3b84a0a3eecc1a9800fed6be9b24e648c161280
Merge "Move flake8 option from run_tests.sh to tox.ini"