[WFLY-1385] Override page-size-bytes default value

The max-size-bytes parameter default value (-1) is overridden in
messaging subsystem configuration to BLOCK at 10MiB.
If the admin changes the address-full-policy from BLOCK to PAGE, this
will conflict with the default value for page-size-bytes (10MiB) which
*must* be smaller than max-size-bytes.
=> override it to 2MiB to avoid any issues when the queues are paging
(and it has no incidence when the queues are blocking)