Code coverage improvement for kolla

This patch add the coverage tool package to test-requirements.txt
which was missing, while kolla support coverage. In addtion to this
tox.ini coverage command is modified to reflect the coverage report
in standard output whenever "tox -e cover" command run for better
representation of coverage report.

Change-Id: I7f77c8d5320a7d180586ce1e8773a4f294846331
Partially-Implements: blueprint coverage-increment-for-kolla

test-requirements.txt

@@ -4,6 +4,7 @@
 bandit>=1.1.0 # Apache-2.0
 bashate>=0.2 # Apache-2.0
 beautifulsoup4 # MIT
+coverage>=4.0 # Apache-2.0
 ddt>=1.0.1 # MIT
 doc8 # Apache-2.0
 extras # MIT

tox.ini

@@ -28,6 +28,7 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_COVER_HTML=1
          NOSE_COVER_HTML_DIR={toxinidir}/cover
 commands = python setup.py testr --coverage --testr-args='{posargs}'
+           coverage report --show-missing
 
 [testenv:pep8]
 commands =