Upgrade coverage tox job

Add ported from Manila project 'cover.sh' script that
allows to calculate coverage diff between last and previous commits.

Usage of new approach (with diff):

$ tox -e cover

To run old coverage approach use following:

./run_tests.sh --coverage --no-pep8

Change-Id: I8f6de6ba5cba2050771c4bead31baaf975e097b9

.coveragerc

@@ -1,7 +1,8 @@
 [run]
 branch = True
 source = manila_ui
-omit = manila_ui/tests/*,manila_ui/openstack/*
+omit = manila_ui/enabled*,manila_ui/test*
 
 [report]
+precision = 2
 ignore_errors = True

tools/cover.sh

@@ -0,0 +1,70 @@
+#!/bin/bash
+#
+# Copyright 2015: Mirantis Inc.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+ALLOWED_EXTRA_MISSING=4
+TESTR_ARGS="$*"
+
+show_diff () {
+    head -1 $1
+    diff -U 0 $1 $2 | sed 1,2d
+}
+
+#
+
+# Stash uncommited changes, checkout master and save coverage report
+uncommited=$(git status --porcelain | grep -v "^??")
+[[ -n $uncommited ]] && git stash > /dev/null
+git checkout HEAD^
+
+baseline_report=$(mktemp -t manila_coverageXXXXXXX)
+find . -type f -name "*.pyc" -delete && ./run_tests.sh --coverage --no-pep8 $TESTR_ARGS
+coverage report > $baseline_report
+baseline_missing=$(awk 'END { print $3 }' $baseline_report)
+
+# Checkout back and unstash uncommited changes (if any)
+git checkout -
+[[ -n $uncommited ]] && git stash pop > /dev/null
+
+# Generate and save coverage report
+current_report=$(mktemp -t manila_coverageXXXXXXX)
+find . -type f -name "*.pyc" -delete && ./run_tests.sh --coverage --no-pep8 $TESTR_ARGS
+coverage report > $current_report
+current_missing=$(awk 'END { print $3 }' $current_report)
+
+# Show coverage details
+allowed_missing=$((baseline_missing+ALLOWED_EXTRA_MISSING))
+
+echo "Allowed to introduce missing lines : ${ALLOWED_EXTRA_MISSING}"
+echo "Missing lines in master            : ${baseline_missing}"
+echo "Missing lines in proposed change   : ${current_missing}"
+
+if [ $allowed_missing -gt $current_missing ]; then
+    if [ $baseline_missing -lt $current_missing ]; then
+        show_diff $baseline_report $current_report
+        echo "I believe you can cover all your code with 100% coverage!"
+    else
+        echo "Thank you! You are awesome! Keep writing unit tests! :)"
+    fi
+    exit_code=0
+else
+    show_diff $baseline_report $current_report
+    echo "Please write more unit tests, we should keep our test coverage :( "
+    exit_code=1
+fi
+
+rm $baseline_report $current_report
+exit $exit_code

tox.ini

@@ -38,7 +38,7 @@ commands = pip install django>=1.8,<1.9
            /bin/bash run_tests.sh -N --no-pep8 {posargs}
 
 [testenv:cover]
-commands = python setup.py testr --coverage --testr-args='{posargs}'
+commands = {toxinidir}/tools/cover.sh {posargs}
 
 [testenv:docs]
 commands = python setup.py build_sphinx