Run patch_neutron function from the correct dir

The patch_neutron function is used in a few places and makes some
assumptions about the current working directory. These assumptions are
breaking other uses of patch_neutron (namely, Jenkins packaging job).
This patch removes the "cd" in the function and moves it into
20-akanda.sh, thus addressing pathing without having to modify anything
else (namely, Jenkins packaging jobs).
It doesn't seem like "changing back" is the prior behavior (though I do think it's probably more correct).  Have you tested this with devstack to make sure it works?