Add a class level store in models/VPCBackend of ec2
for saving vpcs of all regions info. Any instance can correctly find vpc in another region
when connecting vpc of cross-region or vpc of same region.
Modify vpc_peering_connections in ec2/responses to handle
vpc peering of same region or cross region.
Update vpc_peering_connections response
template content to latest (2016-11-15) .
Add vpc cross region peering successful test case.
Add vpc cross region peering fail test case.
Related: https://github.com/spulec/moto/issues/1830
Reference
CreateVpcPeeringConnection Sample Response
https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_CreateVpcPeeringConnection.html
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