Add a class level store in models/VPCPeeringConnectionBackend of ec2
for saving vpc peering connection.
Any instance can correctly save VPC peering connection info
on both region when it create vpc peering connection.
Update vpc_peering_connections in ec2/responses to meet new version:
DESCRIBE_VPC_PEERING_CONNECTIONS_RESPONSE,
ACCEPT_VPC_PEERING_CONNECTION_RESPONSE,
Previous code only create one region VPC peering connection but
doesn't create the other region VPC peering connection
when create cross region VPC peering connection.
Tested in real AWS environment at first
and create unit test case according to real AWS environment response.
Add 5 test cases
VPC cross region delete case
VPC cross region accept case
VPC cross region accept wrong region case
VPC cross region reject case
VPC cross region reject wrong region case
Related: #1842, #1830
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