Don't install zuul in the bindep tox testenv

Running bindep doesn't rely on the zuul source or any of its
dependencies installed, so set usedevelop to False ensuring only
bindep and its dependencies are installed (which is much quicker and
less likely to raise unrelated dependency errors in the process).

Change-Id: I886857e22ed953efdaab5b6247d1e15039806f48

tox.ini

@@ -37,6 +37,7 @@ commands =
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files.
+usedevelop = False
 install_command = pip install {opts} {packages}
 deps = bindep
 commands = bindep test