sql migration: ensure using innodb utf8 for assignment table

During migration from 37 to 38, migration fails to create assignment
table if we don't explictly specify InnoDB and UTF8 charset.

Closes-bug #1320855
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>

Change-Id: Icd88202d2f020aea02514e49c2d07e349c74949f
(cherry picked from commit 18289b9e26ba900302874e72fcbf4529580a973a)
Merge "sql migration: ensure using innodb utf8 for assignment table" into stable/icehouse