Alter logging semantics for VPP PAPI object

Logging previously used a string name for the log level and changed
the system-wide log level based on this string name.  It now uses a
logging-module provided constant for the log level and changes its own
logger's level based on the name, and only if the level is provided.

This allows the logging to be more compatible with Pythonic usage,
where an external source may be used to dictate logging levels across
the system on a per module basis and should not be overridden.

Change-Id: Icf6896ff61a29b12c11d04374767322cdb330323
Signed-off-by: Ian Wells <iawells@cisco.com>