contrail: Switch zookeeper and cassandra from suggest to recommends

Recommends
  This declares a strong, but not absolute, dependency.
  The Recommends field should list packages that would be found
  together with this one in all but unusual installations.

Suggests
  This is used to declare that one package may be more useful with one
  or more others. Using this field tells the packaging system and the user
  that the listed packages are related to this one and can perhaps enhance
  its usefulness, but that installing this one without them is perfectly
  reasonable.

From https://www.debian.org/doc/debian-policy/ch-relationships.html