V2T migration: ensure dvs.ca_file is set for admin shell
Ensure the the ca_file value for validating vCenter server
certificate is alway set when running V2T pre-migration
validation.
The ca_file parameter is set to a particular default value as it
would otherwise default to None
Change-Id: I5b640e47c644dc5c5352b3c7ef80bbb1a7470d34