Externalize the vSphere credentials from the install config file

Feature Overview (aka. Goal Summary)  

Provide the ability to externalize the vSphere username and password.

Goals (aka. expected user outcomes)

As a user I would like to externalize my vSphere username and password to GitHub, Stash, etc or any other credential storage, so that the credentials would not have to be provided as plane texts in the install config file.   

Requirements (aka. Acceptance Criteria):

The user can use the vSphere credentials i.e. username and password  stored outside of the install-config file either form GitHub, Stash, etc or any other credential storage so that the credentials would not have to be provided in the install-config file.  

Use Cases (Optional):

The use cases would be while installing clusters on vSphere using UPI or IPI methods.  

Documentation Considerations

Documentation will be required to explain the user how to externalize the vSphere credentials