Defined "integration.run_tests()" function which can be used to execute
a particular integration test case. Existing bolerplate code in modules
and states tests is replaced with following lines:
if __name__ == '__main__':
from integration import run_tests
run_tests(TestCaseName)
Typical usecase could look like this:
python integration/modules/pip.py --no-clean -vv
I don't really like doing this, but `/tmp/subsalttest dir` is already
hardcoded in other places for running tests.
Without this, when running tests under a mounted Vagrant drive,
`ssh-keygen` doesn't have permission to create the symlink for
known_hosts to known_hosts.old, causing a test to fail.
```
======================================================================
FAIL: test_rm_known_host (integration.modules.ssh.SSHModuleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/vagrant/tests/integration/modules/ssh.py", line 143, in
test_rm_known_host
self.assertEqual(ret, 'add')
AssertionError: 'exists' != 'add'
```
`link /vagrant/tests/integration/tmp/known_hosts to
/vagrant/tests/integration/tmp/known_hosts.old: Operation not permitted`
