Exclude tools/lintstack.head.py for pep8 check
Exclude tools/lintstack.head.py for pep8 check

Commit 062ac3313d7fd39611a6b065ffdff4bf309d8b17 has enabled pep8
check on tools directory, however, if developers have run
`tox -epylint` before, then `tox -epep8` will fail because the
automatically generated file tools/lintstack.head.py cannot pass
pep8 check.

This patch excludes tools/lintstack.head.py in tox pep8 env.

Change-Id: Idbf22821f3d44055f8a03321aa0a05416bec809f

chungg/aodh,cernops/ceilometer,Juniper/ceilometer,Juniper/ceilometer,ityaptin/ceilometer,pczerkas/aodh,isyippee/ceilometer,openstack/ceilometer,mathslinux/ceilometer,chungg/aodh,sileht/aodh,cernops/ceilometer,maestro-hybrid-cloud/ceilometer,fabian4/ceilometer,fabian4/ceilometer,isyippee/ceilometer,r-mibu/ceilometer,idegtiarov/ceilometer,ityaptin/ceilometer,maestro-hybrid-cloud/ceilometer,redhat-openstack/ceilometer,eayunstack/ceilometer,openstack/aodh,eayunstack/ceilometer,mathslinux/ceilometer,pczerkas/aodh,r-mibu/ceilometer,pkilambi/ceilometer,redhat-openstack/ceilometer,idegtiarov/ceilometer,pkilambi/ceilometer,sileht/aodh,openstack/aodh,openstack/ceilometer