tox -ecover failure. Missing entry in tox.ini

`tox -ecover` is failing due to a missing entry in keystone's tox.ini file.
In tox.ini the following code appears:
'''
[testenv:cover]
setenv = NOSE_WITH_COVERAGE=1
         NOSE_COVER_HTML=1
         NOSE_COVER_HTML_DIR={toxinidir}/cover
'''

Adding 'VIRTUAL_ENV={envdir}' to the setenv mapping, as shown below,
has resolved the issue on my machine.
'''
[testenv:cover]
setenv = VIRTUAL_ENV={envdir}
         NOSE_WITH_COVERAGE=1
         NOSE_COVER_HTML=1
         NOSE_COVER_HTML_DIR={toxinidir}/cover
'''

Change-Id: I3550ac61cdc6389dd239c5ad319d881519b3543d
Fixes: bug #1195371

tox.ini

@@ -23,7 +23,8 @@ commands =
 downloadcache = ~/cache/pip
 
 [testenv:cover]
-setenv = NOSE_WITH_COVERAGE=1
+setenv = VIRTUAL_ENV={envdir}
+         NOSE_WITH_COVERAGE=1
          NOSE_COVER_HTML=1
          NOSE_COVER_HTML_DIR={toxinidir}/cover