logging: Stringify message in StepFormatter

The usage of StepFormatter could provide errors:

    self = <labgrid.logging.StepFormatter object at 0x7f63fdc59010>
    record = <LogRecord: foo, 30, .../test_logger.py, 10, "{1: 2}">

        def format(self, record):
            old_msg = record.msg
            try:
                if self.indent:
                    if hasattr(record, "indent_level"):
                        self.indent_level = record.indent_level

    >               record.msg = (" " * self.indent_level) + record.msg
    E               TypeError: can only concatenate str (not "dict") to str

if a message isn't a string.

Change-Id: I0f5187dcd9389ff486aac2f7e0e029ecaa323825

labgrid/logging.py

@@ -37,7 +37,7 @@ def format(self, record):
                 if hasattr(record, "indent_level"):
                     self.indent_level = record.indent_level
 
-                record.msg = (" " * self.indent_level) + record.msg
+                record.msg = (" " * self.indent_level) + str(record.msg)
 
                 self.indent_level = getattr(
                     record, "next_indent_level", self.indent_level