Fix slice expiration check in renewsliver.
Fix slice expiration check in renewsliver.

renewsliver erroneously checked the slice expiration date. Fix it to
use _has_slice_expired utility instead. Use the slice expiration
returned by _has_slice_expired instead of asking the credential for
the expiration and converting it to naive UTC over again.

GENI-NSF/gram,GENI-NSF/gram,GENI-NSF/gram,tcmitchell/geni-tools,ahelsing/geni-tools,plantigrade/geni-tools,plantigrade/geni-tools,tcmitchell/geni-tools,ahelsing/geni-tools