VMware host drivers for Fusion, vSphere and vCloud Air

This is a follow up on #18.

These are our initial contribution to the host management functionality, a couple of notes:

- vSphere driver depends on vmware/govmomi/govc (must be in path) and works with vmware/govmomi@c8486300bfe19427e4f3226e3b3eac067717ef17 
- vCloud Air driver introduces a dependency on vmware/govcloudair@1df64d51cbbac795bdeeab8b4030cdf2e0471ccd 

Thanks!