Change the instance status attribute to boolean

Due to historic reasons, the “should run or not” attribute of an
instance was denoted by its “status” attribute having a string value of
either ‘up’ or ‘down’. Checking this is in code was done via hardcoding
of the strings.

This was long done for a redo, and this patch changes this attribute to
“admin_up” having a boolean value. The patch is in fact shorter than I
expected, and passes burnin.

The patch also fixes an error in BuildInstanceHookEnvByObject where the
instance.os was passed as the status value.

Reviewed-by: ultrotter