Python Logging Formatting Improvements

The Open vSwitch daemons written in C support user-configured logging
patterns as described in ovs-appctl(8). This commit adds this capability
to the daemons written in Python.

- Add a '__log_patterns' attribute to the Vlog class
- Populate this using the default patterns in ovs-appctl(8)
- Add a '__start_time' attribute to the Vlog class to support '%r'
- Update the '_log' method to build the log message according to the
  pattern
- Add a 'set_pattern' method to allow the default patterns to be changed
- Update 'set_levels_from_string' to support setting the pattern from a
  string

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Ben Pfaff <blp@nicira.com>