Slight changes to Vagrant developer configs

This makes two changes to the Vagrant defaults for the developer VM:
- Lowers VM memory from 2048 to 512, and pins the VM to 25% of one CPU
  core. This is helpful for low-power development environments (like
  laptops) and is sufficient for RabbitMQ and MySQL.
- Changes the default set of hardware drivers to be a minimal set
  for developer environments that do not require the installation
  of any hardware-specific utilities or libraries.
  This list is: pxe_ssh, agent_ssh, fake

Change-Id: Icb5e0e42a07386b93250f8fa461424b69c7cdee9

Vagrantfile

@@ -9,7 +9,7 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
 
   config.vm.define 'ironic' do |ironic|
     ironic.vm.provider :virtualbox do |vb|
-      vb.customize ['modifyvm', :id,'--memory', '2048']
+      vb.customize ['modifyvm', :id, '--memory', '512', '--cpuexecutioncap', '25']
     end
 
     ironic.vm.network 'private_network', ip: '192.168.99.11' # It goes to 11.

vagrant.yml

@@ -116,7 +116,9 @@
           }
         - {
             section: 'DEFAULT',
-            option: 'enabled_drivers', value: 'fake_ipmitool'
+            option: 'enabled_drivers', value: 'pxe_ssh, agent_ssh, fake'
+            # All other testing drivers require add'l packages
+            # and should be enabled locally, if desired
           }
         - {
             section: 'DEFAULT',