clientconfig: Support vendor profiles

[outcatcher]

Currently, there is a list of default vendor profiles that simplifies the configuration of cloud entities with clouds.yaml configuration file. It's part of the python SDK, so supported by all python-based tools, including python-openstackclient.

As it's missing from the utils, so the behavior of the Go-based tools which is using them defers from the standard one.

Vendor support is described in the documentation at https://docs.openstack.org/openstacksdk/latest/user/config/vendor-support.html. As configuration file support is a part of utils, I suppose it belongs there.

If you approve this feature request, I can make a PR myself.