Add vpc-cidr parameter to convox install for custom vpc sizes

[cleblanc87]

This PR adds the ability to specify the size of the VPC you want to allocate for a convox rack. The subnets are calculated dynamically by splitting the chosen vpc into the 4 largest subnets that will fit.

[csquared]

parameterize subnets in rack

[csquared]

@cleblanc87 this looks great! i'm going to cut a branch release that either of us can install to test. in the meantime i have two requests:

1. make a quick gofmt pass to get all the spaces uniform
2. make the subnet calculation its own function that we can write a test for. (thinking something simple in cmd/convox/install_test.go

happy to assist in any way with the above

[csquared]

just did a convox install --version 20151201171730-cleblanc87-custom-cidr ...

Looks good!

[cleblanc87]

Removed subnet calculation in favor of cli parameters, for simplicities sake

[csquared]

@cleblanc87 if you don't mind pulling master and rebasing i'd love to merge this in!

[csquared]

thanks @cleblanc87