mkcloud: add option to set the memory assigned to admin and compute nodes

The following new environment variables are used:

* ADMIN_NODE_MEMORY to set the memory in KB assigned to admin node
* COMPUTE_NODE_MEMORY to set the memory in KB assigned to compute nodes

By default both are set to 2097152 (2GB)
Add support to reboot exclusively neutron controller

Executing `./mkcloud rebootneutron` will reboot the node where
`neutron-server` is running.

This is useful to test how the cloud will behave after rebooting the network,
an expected use case for this is running the following command:

    # ./mkcloud all_noreboot rebootneutron testsetup
Added support to configure cloud using ovs+vlan

To configure neutron using openvswitch with VLAN support set environment
variables `networkingplugin` to `openvswitch` and `networkingmode` to
`vlan`.