Do not refresh timestamp from database on read.

Doing a database refresh on timestamp read is causing errors
such as:
heat.openstack.common.rpc.amqp InvalidRequestError:
  Could not refresh instance '<Stack at 0x2c95a90>'

This has only been seen using postgres, possibly because the problem is
being masked on mysql due to the driver's lack of concurrency.

The database refresh seems to be unnecessary, as there appears to be
no logic which depends on timestamps being accurate (nor is there
any timestamp comparison logic at all)

Fixes bug: #1193132
Change-Id: I22c3c4546a0f44b76a95e473b68b6fbb2e423b90