Add python-neutronclient to requirments for tests

Import of python-neutronclient was added to
system tests by commit to fuel-main #
5e4ae8d3f38ecbf0af3959651cbacfd40ec8f299, so
it should be added to requirements

Change-Id: I49108278e365c20bec5de5c595992fc4a68e1a07
Closes-bug: #1330615
Merge "Add python-neutronclient to requirments for tests"