This resource allows an existing Route Table to be assigned as the
"main" Route Table of a VPC. This means that the Route Table will be
used for any subnets within the VPC without an explicit Route Table
assigned [1].
This is particularly useful in getting an Internet Gateway in place as
the default for a VPC, since the automatically created Main Route Table
does not have one [2].
Note that this resource is an abstraction over an association and does not
map directly to a CRUD-able object in AWS. In order to retain a coherent
"Delete" operation for this resource, we remember the ID of the AWS-created
Route Table and reset the VPC's main Route Table to it when this
resource is deleted.
refs #843, #748
[1] http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Route_Tables.html#RouteTableDetails
[2] http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Internet_Gateway.html#Add_IGW_Routing
