Compatible server default value in the models.

MySQL has no native boolean type. MySQL boolean type is just an alias
to tiny int type field. So MySQL returns '"1"' for `true` value and
'"0"' for false value. Instead of it, Postgres has native boolean type.
Thus with Postgres, reflection of a boolean type column default value will
return either 'true' or 'false'. Model must have compatible
`server_default` argument in columns definition.

Change-Id: Id0107d510dd7087e61ae851a1f3ff18334851a68
Partial-Bug: #1292591
Merge "Compatible server default value in the models."