Increase default sizing of development VM.
Increase default sizing of development VM.

Set memory to 1024M and cpus to 2 by default.  Without setting this, the
VM gets the virtualbox defaults of 384M RAM, and 1 cpu.  This is not
enough to run even part of the stack.

These new default values can still be overridden by `Vagrantfile.local`

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet