tests fail in fedora 18 due a requirements problem

In a fresh installation of Fedora 18, when trying to run the tests with tox a dependencies problems in python packages makes them fail.

The problem lies on pip's inability to process properly recursive requirements and deciding that pyparsing's version from the system (1.5.6) is fine while cliff (a python-quantumclient's requirement) needs at least 1.5.7.

In pip's issue tracker (See https://github.com/pypa/pip/issues/988) they state that the best solution is to set in the main project the min/max version needed to prevent such problem.

nova $ tox -e py27

[...]

Could not load 'keypairs': (pyparsing 1.5.6 (/usr/lib/python2.7/site-packages), Requirement.parse('pyparsing==1.5.7'))
(pyparsing 1.5.6 (/usr/lib/python2.7/site-packages), Requirement.parse('pyparsing==1.5.7'))
Traceback (most recent call last):
  File "/home/xqueralt/w/r/c/nova/.tox/py27/lib/python2.7/site-packages/stevedore/extension.py", line 83, in _load_plugins
    invoke_kwds,
  File "/home/xqueralt/w/r/c/nova/.tox/py27/lib/python2.7/site-packages/stevedore/enabled.py", line 46, in _load_one_plugin
    ep, invoke_on_load, invoke_args, invoke_kwds,
  File "/home/xqueralt/w/r/c/nova/.tox/py27/lib/python2.7/site-packages/stevedore/extension.py", line 95, in _load_one_plugin
    plugin = ep.load()
  File "/home/xqueralt/w/r/c/nova/.tox/py27/lib/python2.7/site-packages/distribute-0.6.34-py2.7.egg/pkg_resources.py", line 2012, in load
    if require: self.require(env, installer)
  File "/home/xqueralt/w/r/c/nova/.tox/py27/lib/python2.7/site-packages/distribute-0.6.34-py2.7.egg/pkg_resources.py", line 2025, in require
    working_set.resolve(self.dist.requires(self.extras),env,installer))
  File "/home/xqueralt/w/r/c/nova/.tox/py27/lib/python2.7/site-packages/distribute-0.6.34-py2.7.egg/pkg_resources.py", line 598, in resolve
    raise VersionConflict(dist,req) # XXX put more info here
VersionConflict: (pyparsing 1.5.6 (/usr/lib/python2.7/site-packages), Requirement.parse('pyparsing==1.5.7'))

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