Address the EC2 limitation on maximum number of VPCs

Description:
We need to address the EC2 limitations such as the number of VPCs possible. We can immediately start addressing the issue with # of VPCs and internet gateways. See the image below:

We need to come up with a strategy on how to address this limitation. Immediate options:

1. Request AWS support to increase the limitation of 5 (to may be 20?). We get this done for multiple regions, not just for us-east-1.
2. Leverage AWS regions to workaround this limitation. We can create five VPCs in each of the regions. There are total of 10+ regions. If we leverage this, we can go for a maximum of 50 (5VPCs * 10 regions) parallel deployments.
   If the per-region limitation was increased from 5 to 20 for each region, then the total parallel deployments possible become 200 (20 * 10).
3. Leverage subnets within existing VPCs. Each VPC can have 255 different subnets. Testgrid should be able to leverage this.
   To do this, we have to update the existing cfn scripts to use an existing VPC (by taking it as a input parameter) instead of creating a new VPC for every new deployment. We can pre-process the cfn templates from testgrid side in order to do this.

Out of the above three, we should immediately start doing #1.
At the same time, we should start doing #2 in parallel. This implementation will allow testgrid to be scalable. So, this is not just about addressing VPC count limitation.

Option #3 is also important. But, I think we can live with AWS regions for now.

Affected Product Version:
0.9.0-m14

OS, DB, other environment details and versions:
N/A

Steps to reproduce:
N/A

Related Issues:
#649