Currently SNs report their free disk space once per hour. If a node
becomes full, it has to wait until the next contact cycle begins to
report; all the while receiving and failing upload requests. By increasing
the minimum required disk space, we can give the storage nodes more time
to report their space before the completely fill up. This change goes
hand-in-hand with another change we want to implement: trigger capacity
report on SN immediately upon falling below threshold.
Change-Id: I12f778286c6c3f582438b0e2949765ac43325e27
b22bf16b35