VPC route table assocation handling update - EUCA-10107

Route table assocations are now persisted in a new RouteTableAssociation
entity. The main route table for a vpc can now be altered using the
ReplaceRouteTableAssociation action. DescribeRouteTables is updated to
show the main association correctly.

EUCA-10107
Merge remote-tracking branch 'origin/dev/sjones/EUCA-10107' into testing