Allow function patch_neutron to accept a directory

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 allows the patch_neutron function to be called with a
parameter, thus allowing a default action (of "cd $DEST/neutron") or
allowing the user to override the default (by calling the function by:
patch_neutron /var/tmp).

This may (will) required additional changes to anything using this function
that needs to call the function differently (namely, Jenkins packaging jobs).