* Use unix:// prefix for CSI_ENDPOINT variable by default
* Some plugins have strict validation over the format of the
`CSI_ENDPOINT` variable, and unfortunately not all plugins
agree. Allow the user to override the `CSI_ENDPOINT` to workaround
those cases.
* Update all demos and tests with CSI_ENDPOINT
A demo for the CSI hostpath plugin, used primarily for development
purposes. This reproduction has been used for testing during our CSI
implementation work, and this changeset makes that public.