builder/vmware: new driver to support building images directly on ESX

This driver talks directly to ESX over ssh, using vim-cmd, esxcli and sh;
no vCenter or VIM api required.

Remote* config properties added to support a remote driver

RemoteDriver interface extends Driver:
* SSHAddress - esx flavor uses esxcli to find the VM's ip address
* Download - esx flavor downloads iso files to a vmfs datastore

Driver can optionally implement the following interfaces:
* VNCAddressFinder - esx flavor needs to check remote ports
* OutputDir - esx driver needs a local and remote OutputDir
* Inventory - esx driver needs to register/unregister VMs
* HostIPFinder - esx flavor needs an address on the same network as esx itself
builder/vmware: make things more Go-like

This commit currently breaks the builder though, since the ISo is now
uploaded back into ESX.