Make AllocateResponse artifacts global across all devices per container in device plugin API

There is no use case known for passing artifacts per device as it currently exists. The current API is also
complex to use for simple clients. Hence this PR creates a flat namespace where artifacts like environment variables
and mount points apply globally to all devices returned as part of AllocateResponse proto.

Signed-off-by: Vishnu kannan <[email protected]>

Author: vishh