Externalize vSphere credentials to avoid plaintexts while installing clusters on VMware

Issue

• Currently it is not possible to externally provide the VMware vSphere username and password to the install-config.yaml file used to install IPI clusters on VMware vSphere.
• The credentials have to be provided as plaintexts in the install-config.yaml file which can be a security risk.
• It is not possible to provide encrypted credentials a well in the install-config file.