Add a table and skeltons for ip retention container
Merge branch 'master' into feature-ip-retention-container

Conflicts:
	vnet/db/migrations/0001_origin.rb
Update events and node api
Refactor ip_retention_container.rb
Add Vnet::NodeApi::IpLease.expire
Merge branch 'master' into feature-ip-retention-container
Replace IpRetentionManager with IpRetentionContanerManager
Update IpRetentionContainerManager and implement its spec
Set lease_time_expired_at on before_create
Add spec for NodeApi::IpRetentionContainer
Implement endpoint for ip_retention_container
Add GET '/lease_policies/:uuid/ip_leases'
Fix a manager name
Refactor based on review comments
Add comments on event handlers.
Refactor check_expiration
Update relationship: lease_policies many_to_many ip_retention_containers
Change ip_retention table
Update ip_retention_container/base.rb
Update ip_retention_container_manager.rb