Change subnet for dev bootstrap to us-east-1b
Change subnet for dev bootstrap to us-east-1b

We've been running into the following error fairly regularly when
creating bootstrap instances:

    InsufficientInstanceCapacity => We currently do not have sufficient
    m3.large capacity in the Availability Zone you requested (us-east-1e).
    Our system will be working on provisioning additional capacity. You can
    currently get m3.large capacity by not specifying an Availability Zone
    in your request or choosing us-east-1b, us-east-1c, us-east-1a.

Using a different AZ (which is determined by the subnet being used)
normally avoids this, which suggests that the capacity problems are
limited to a single AZ.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf