Mm feature/fix export vm as ovf vars

[mikemorency]

This change is from the testathon feedback.

It includes two issues:

1. The documentation for the export_vm_as_ovf role had duplicate and unused variables defined
2. The role uses a _name suffix on the datacenter variable that is inconsistent with other roles.

I updated the docs and changed the variable to match other roles, while keeping the old variable name valid

[anna-savina]

LGTM