Test min coverage not to drop down

Note:
Later min percentage should be raised up.

Must be added to
openstack-infra/project-config/blob/master/zuul/layout.yaml
check:
    - swauth-coverage
post:
    - swauth-coverage

Change-Id: I3cb9d52350ad5fe63faf7f332f0bb784d870ab46

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = py27,pep8
+envlist = py27,pep8,cover
 skipsdist = True
 
 [testenv]
@@ -10,6 +10,7 @@ install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          NOSE_WITH_COVERAGE=1
          NOSE_COVER_BRANCHES=1
+         NOSE_COVER_ERASE=1
 deps =
     -r{toxinidir}/test-requirements.txt
     https://launchpad.net/swift/kilo/2.3.0/+download/swift-2.3.0.tar.gz
@@ -21,6 +22,8 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_COVER_BRANCHES=1
          NOSE_COVER_HTML=1
          NOSE_COVER_HTML_DIR={toxinidir}/cover
+         NOSE_COVER_MIN_PERCENTAGE=85
+         NOSE_COVER_ERASE=1
 
 [testenv:pep8]
 commands =