Port test_nfs to Python 3

Port test_nfs to Python 3

testtools.ExpectedException(exc_class, regex) expects the exact
exc_class class, wheras the
test_setup_should_throw_exception_if_mount_nfs_command_fails() test
raises a PermissionError on Python 3.3+ instead of OSError. Replace
testtools.ExpectedException() with self.assertRaisesRegex() to port the
test to Python 3, this method accepts subclasses of exc_class.

Replace also testtools.ExpectedException() with self.assertRaisesRegex()
in other tests.

Blueprint https://blueprints.launchpad.net/cinder/+spec/cinder-python3
Change-Id: If5a74529e5ac68a2118afa8ecfd86bf24307b0b7