dockertest.subtest: Reformat logging and fix couple of bugs

This pull request is extension of PR https://github.com/autotest/autotest-docker/pull/145

This patch changes the logging method to respect these rules:
1) debug - copy&paste friendly, not necessarily nice
2) warning,error,info - manager frienly, emphasize multiline messages
                        by space-based indentation

By design this changes the issues of silently ignored messages when
lines with arguments were used (eg. `self.loginfo("test %s", 1)`)

Sample code:
```python
        for log in (self.logdebug, self.logerror, self.loginfo, self.logwarning):
            log("bad %s", "guy")
            log("aaa%sbbb\nccc", '\n')
```

Original
```
17:25:23 INFO |                 no_wait: initialize()
17:25:23 DEBUG|                 no_wait: bad guy
17:25:23 DEBUG|                 no_wait: aaa
bbb
                                ccc
17:25:23 ERROR|                 no_wait: aaa
17:25:23 ERROR|                 no_wait: bbb
17:25:23 ERROR|                 no_wait: ccc
17:25:23 INFO |                 no_wait: aaa
17:25:23 INFO |                 no_wait: bbb
17:25:23 INFO |                 no_wait: ccc
17:25:23 WARNI|                 no_wait: aaa
17:25:23 WARNI|                 no_wait: bbb
17:25:23 WARNI|                 no_wait: ccc
```

New version:
```
17:25:55 INFO |                 no_wait: initialize()
17:25:55 DEBUG|                 no_wait: bad guy
17:25:55 DEBUG|                 no_wait: aaa
bbb
ccc
17:25:55 ERROR|                 no_wait: bad guy
17:25:55 ERROR|                 no_wait: aaa
                                         bbb
                                         ccc
17:25:55 INFO |                 no_wait: bad guy
17:25:55 INFO |                 no_wait: aaa
                                         bbb
                                         ccc
17:25:55 WARNI|                 no_wait: bad guy
17:25:55 WARNI|                 no_wait: aaa
                                         bbb
                                         ccc
```
If we make the work done by this one line into another @classmethod, then it could be overridden by the SubSubtest class to include the name of the parent sub-subtest.  This could help eliminate any/all confusion with multi-line messages where indentation in message clashes (visually) with sub-subtest indentation.