Sync latest db and importutils from oslo

Changes include:

  7c0e25a Use importutils.try_import() for MySQLdb
  33b12d3 Implement importutils.try_import.
  cf705c5 Make project pyflakes clean.

Change-Id: I448f5208919d06ca0af54d2057b8f5e042862cd6
Merge "Sync latest db and importutils from oslo"