Add keyword to manipulate vhost-user parameters

JIRA: CSIT-70

- add a keyword to be able to:
  - configure all vhost-user parameters at once
  - configure vhost-user parameters one by one
  - remove a vhost-user parameter
  - remove all vhost-user parameters at once
- add a keyword which adds a new vhost-user interface

Change-Id: I144bc035b959a1e4eb37c6a3d554e6843ddab4a7
Signed-off-by: Tibor Frank <tifrank@cisco.com>