Stop depending on sitepackages libvirt-python

We can install libvirt-python bindings from pypi now because the libvirt
devs are supporting pypi packages and the version of libvirt we are
testing nova master against is new enough to link against that pypi
package.

Making this switch will better isolate nova test environments as all
dependencies can be installed without any site packages leaking into
virtualenvs. It will also make the dependency on libvirt-python a bit
more explicit.

Change-Id: I2890702869f05a02ad5e2ecb419db06433231b36

test-requirements.txt

@@ -3,6 +3,7 @@ coverage>=3.6
 discover
 feedparser
 fixtures>=0.3.14
+libvirt-python>=1.2.5
 mock>=1.0
 mox>=0.5.3
 MySQL-python

tox.ini

@@ -4,7 +4,6 @@ envlist = py26,py27,py33,pep8
 skipsdist = True
 
 [testenv]
-sitepackages = True
 usedevelop = True
 install_command = pip install -U --force-reinstall {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
@@ -14,16 +13,13 @@ commands =
   python -m nova.openstack.common.lockutils python setup.py test --slowest --testr-args='{posargs}'
 
 [tox:jenkins]
-sitepackages = True
 downloadcache = ~/cache/pip
 
 [testenv:pep8]
-sitepackages = False
 commands =
   flake8 {posargs}
 
 [testenv:genconfig]
-sitepackages = False
 commands =
   bash tools/config/generate_sample.sh -b . -p nova -o etc/nova
 
@@ -48,9 +44,6 @@ commands = {posargs}
 [testenv:docs]
 commands = python setup.py build_sphinx
 
-[testenv:py27local]
-sitepackages = False
-
 [flake8]
 # H803 skipped on purpose per list discussion.
 # E125 is deliberately excluded. See https://github.com/jcrocholl/pep8/issues/126