Replace hard coding of integration interface with attribute
Default agent_down_time to 30s

Add an attribute for agent_down_time and default to 30 seconds.

Neutron's default of 5 seconds is far too aggressive when checking
agent status, especially as the number of networks increases.
Merge pull request #63 from DavidWittman/agent-down-time

Merged automatically by jenkins after successful build test
Merge pull request #62 from odyssey4me/DE251

Replace hard coding of integration interface with attribute