Docstrings formatted according to pep257

Bug #1020184

quantum/plugins/hyperv/*
quantum/plugins/linuxbridge/*
quantum/plugins/nec/*
quantum/plugins/nicira/*
quantum/plugins/plumgrid/*
quantum/plugins/ryu/*
quantum/plugins/services/*

Change-Id: I3525aa57235935d90ec7b70c65ba15012ce575a3
Merge "Docstrings formatted according to pep257"