os: update vmware guide
os: update vmware guide

Users should be using the standard VMware images. This suggests that
they use the OVA image so it can be directly imported into ESXi.

bq-xiao/docs,michelleN/docs,hkjn/docs-1,marineam/coreos-docs,joshix/docs,atxwebs/docs,coreos/docs,jdloft/docs,robszumski/docs,jonboulle/docs,mjg59/docs,endocode/coreos-docs,wli6/docs