Use python abc in StressAction class

Fix pylint error about NotImplemented raised instead of
NotImplementedError.

Use the abc class instead of NotImplementedError.

Partial-Bug: #1346797

Change-Id: Ia9a75a53f68c28594654a07146a2a30c1ae3c412
Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge "Use python abc in StressAction class"
Merge branch 'master' of https://github.com/openstack/tempest