Update vcsa link in README

Previously, the link to the vcsa module
refers to a module from puppetlabs
github account that no longer exists.

This commit updates the link location to
point at the vmware version of the module.
Merge pull request #137 from bodepd/update_vcsa_href

Update vcsa link in README
Provide proper error message when no esx compatible for DVS.
Merge pull request #139 from nanliu/dvs

Provide proper error message when no esx compatible for DVS.