Add ssh-keyscan to unblock kolla-ansible gate

As a change to remove puppet from infra nodepool,
ssh-keyscan is needed for each node.[0]

Host key checking was disabled by default by jenkins
user. [1]

Refs:
[0] http://lists.openstack.org/pipermail/openstack-dev/2017-April/116036.html
[1] https://review.openstack.org/#/c/460731/
Change-Id: I0f4000f18b7a2b3266c95c3f7a5109e3603c94a7