Change report_interval from 4 to 30, agent_down_time from 9 to 75

Change report_interval from 4 to 30, agent_down_time from 9 to 75

report_interval is how often an agent sends out a heartbeat to the
service. The Neutron service responds to these 'report_state' RPC
messages by updating the agent's heartbeat DB record.
The last heartbeat is then compared to the configured
agent_down_time to determine if the agent is up or down.
The agent's status is used when scheduling networks on DHCP
and L3 agents.

In the spirit of sane defaults suited for production, these values
should be bumped to reduce the load on the Neutron service
dramatically, freeing up CPU time to perform intensive operations.

DocImpact
Closes-Bug: #1293083
Change-Id: I77bcf8f66f74ba55513c989caead1f96c92b9832