Fix coverage report

There's a few things going on here

Firstly, we need to install coverage tool in test-requirements

Secondly, .testr.conf has to use PYTHON because the coverage report
works by resetting PYTHON='coverage run ...'

Thirdly, because we call ourselves diskimage-builder but the python
module is diskimage_builder that seems to confuse things.  We need to
use "setup.py test" (note, that is different to "setup.py testr"!) to
use the PBR testr wrapper.  That exposes a --coverage-package-name
argument that calls the coverage tool with the right argument.

With this I got a coverage report for our unit tests

Change-Id: I9012e18eb7d01bee035140e70afa76c47c27eb01

.testr.conf

@@ -4,7 +4,7 @@ test_command=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
              OS_LOG_CAPTURE=${OS_LOG_CAPTURE:-1} \
              OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} \
              OS_DEBUG=${OS_DEBUG:-0} \
-             python -m subunit.run discover . $LISTOPT $IDOPTION
+             ${PYTHON:-python} -m subunit.run discover . $LISTOPT $IDOPTION
 test_id_option=--load-list $IDFILE
 test_list_option=--list
 

test-requirements.txt

@@ -12,3 +12,5 @@ oslosphinx>=4.7.0 # Apache-2.0
 
 # releasenotes
 reno>=1.8.0 # Apache-2.0
+
+coverage>=4.0 # Apache-2.0

tox.ini

@@ -9,7 +9,7 @@ install_command = pip install -U {opts} {packages}
 deps= -r{toxinidir}/requirements.txt
       -r{toxinidir}/test-requirements.txt
 commands=
-    python setup.py testr --slowest --testr-args='{posargs}'
+    python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
 commands =
@@ -24,9 +24,10 @@ envdir = {toxworkdir}/venv
 commands = {toxinidir}/tests/run_functests.sh {posargs}
 
 [testenv:cover]
-setenv = PYTHON=coverage run --source diskimage_builder
-commands = bash -c 'if [ ! -d ./.testrepository ] ; then testr init ; fi'
-           bash -c 'testr run --parallel ; RET=$? ; coverage combine ; coverage html -d ./cover $OMIT && exit $RET'
+# NOTE: this is "setup.py test" (*not* testr) which is a pbr wrapper
+# around testr.  This understands --coverage-package-name which we
+# need due to underscore issues.
+commands = python setup.py test --coverage --coverage-package-name diskimage_builder --testr-args='{posargs}'
 
 [testenv:docs]
 commands = python setup.py build_sphinx