Add el6 distro builds
Add el6 distro builds

Since https://github.com/StackStorm/st2-packages/pull/65 is complete, start building `el6` packages.
Plexxi/st2,dennybaa/st2,nzlosh/st2,dennybaa/st2,Plexxi/st2,punalpatel/st2,nzlosh/st2,peak6/st2,emedvedev/st2,lakshmi-kannan/st2,lakshmi-kannan/st2,StackStorm/st2,lakshmi-kannan/st2,Plexxi/st2,StackStorm/st2,armab/st2,punalpatel/st2,tonybaloney/st2,Plexxi/st2,emedvedev/st2,dennybaa/st2,punalpatel/st2,armab/st2,nzlosh/st2,peak6/st2,pixelrebel/st2,StackStorm/st2,emedvedev/st2,tonybaloney/st2,tonybaloney/st2,nzlosh/st2,pixelrebel/st2,StackStorm/st2,peak6/st2,armab/st2,pixelrebel/st2