Add sync to avoid the loss of pub key data
Add sync to avoid the loss of pub key data

On the hard rebooting test, the pub key data can be loss due to the
immediate rebooting. In addition, the existing scenario test manager
is using 'sync' command to avoid the other data loss.
So this patch adds a 'sync' command call.

Change-Id: Ie55ad394cdbbde100eee45026c1e52a6f2825b77
Closes-Bug: #1583987

Juniper/tempest,vedujoshi/tempest,sebrandon1/tempest,Juniper/tempest,cisco-openstack/tempest,openstack/tempest,sebrandon1/tempest,masayukig/tempest,openstack/tempest,masayukig/tempest,cisco-openstack/tempest,Tesora/tesora-tempest,vedujoshi/tempest,Tesora/tesora-tempest