Fix check for memory_mb
Fix check for memory_mb

assert_compute_node_has_enough_memory should check memory_mb is None or not

_assert_dest_node_has_enough_disk should check available_mb as well

Fixes bug 884842

Change-Id: Ieaa33efd50bb84f1b475c437b26af1302fdfd19f

n0ano/gantt,n0ano/gantt