add vsphere driver support

This enables  for boot2docker CLI. For the requirements/configuration to enable the vSphere bindings, please refer to the README file.

The test has been done on Ubuntu, Windows and Mac using go cross compile, and is working against vSphere 5.5.

For the govc dependencies, the minimal commit is vmware/govmomi@4d5ed8741fa33b45eb7afdde36f18ab364e2790a. For boot2docker ISO dependencies, the minimal dependency is vmware/boot2docker@d3603d921cd0a374d68b7bc9f36adfa1522f5b36 for ovt branch.