Add tox env for checking links in docs

Change-Id: I99c924c7cdec488475f32099193cba53ed6afb4c

tox.ini

@@ -25,6 +25,12 @@ commands =
     python setup.py build_manpage
     python setup.py build_sphinx {posargs}
 
+[testenv:docs-linkcheck]
+# If you are behind a proxy, for this test to work you will need to set
+# TOX_TESTENV_PASSENV="http_proxy https_proxy no_proxy ..." to pass
+# through the proxy environment settings to be able to validate any urls.
+commands = python setup.py build_sphinx -b linkcheck
+
 [testenv:venv]
 commands = {posargs}