Improved installation guide NTP section

I improved the NTP section of the installation guide as follows:

1) Changed configuration of other nodes to reference controller
   node.
2) Added section to verify synchronization of all nodes.

Change-Id: I2de93426b3cf82a100a0b217f1272fc0e59ddb92
Implements: blueprint installation-guide-improvements
Merge "Improved installation guide NTP section"