Integration test should support subunit output

[Andreas Hasenack]

The charm integration test should support a subunit output, so that test results can be parsed by jenkins.

As a POC, I tried this, and it worked:
=== modified file 'tests/helpers.py'
--- tests/helpers.py 2015-07-10 07:54:47 +0000
+++ tests/helpers.py 2015-07-14 21:39:30 +0000
@@ -606,6 +608,7 @@
     module = os.path.basename(sys.argv[0]).split("-")[1]
     args = sys.argv[:]
     args.extend(["-vv", "--path", path, "--tests-pattern", "^%s$" % module])
+ args.append("--subunit")
     run(args=args)