Fix tox coverage job

To run the tempest unit tests with coverage enabled the coverage
module needs to be installed. This requirement is properly listed in
test-requirements.txt, however the coverage job definition wasn't
installing coverage. This commit fixes this oversight by adding the
test-requirements file as a dependency for the job.

Change-Id: Ic443c6c4a3166e6b198cbe2af0a77ff2f0a38e83
diff --git a/tox.ini b/tox.ini
index a071d4b..6ec0b2c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -29,6 +29,8 @@
 setenv = OS_TEST_PATH=./tempest/tests
          PYTHONHASHSEED=0
 commands = python setup.py testr --coverage --testr-arg='tempest\.tests {posargs}'
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
 
 [testenv:all]
 sitepackages = True