Added requests into nailgun/requirements.txt

There is also a patch to fuel-main which adds
python-requests to nailgun package spec.

fuel-main patch: I280de416af0079da1976ab37112dbf8764ef2839

Change-Id: I73db9cd7724291dc57904834d38dbe9afb371462
Closes-Bug: 1393859
Merge "Added requests into nailgun/requirements.txt"