To make it easier to later move to oslo.db match there ability
to set a different mysql mode (this will default, unless configured
explicitly off/differently to TRADITIONAL).
To ensure that we retain backwards compatibility the prior existence
of 'mysql_traditional_mode' as a boolean option will by default set
the mode to TRADITIONAL, and a new configuration option 'mysql_sql_mode' can
be used to provide a secondary overriding mode that will be used
instead (incase someone wants to use something other than TRADITIONAL).
Closes-Bug: 1326568
Change-Id: Ide34c27b12c26030c8842f3f4b0fcca43ce783a7