Reduce default polling interval to 30 seconds

The initial value of 300 seconds was a conservative estimate. However,
the requests and responses are somewhat small, so we can drop the polling
interval significantly and still maintain a high degree of network
scalability. After measuring the responses from the ec2 and cfn servers
with typical workloads, at 30 second intervals 100 servers will generate
around 26kB/s of requests, with about 66kB/s of responses.

Change-Id: Iaa99ae405ba7c72ef8afc11c946400a2d0db5206