dockertest.subtest: Reformat logging and fix couple of bugs

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)`)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>