Use groups instead of users for WebAPI and VNMGR

[Metallion]

This is related to issue https://github.com/axsh/openvnet/issues/28.

Problem

In https://github.com/axsh/openvnet/pull/299 we changed WebAPI and VNMGR to run as their own user. After a discussion with @akry, it was decided that it is better to use groups. All users in the groups should be able to start their respective services.

Solution

• Create 2 groups. vnet-webapi and vnet-vnmgr.
• Keep the users created in https://github.com/axsh/openvnet/pull/299 but add them to their respective groups.
• In the upstart job, check for the correct group.
  • If part of the correct group, start the process.
  • if root, start the process as the default user. (the one set as VNET_USER in the /etc/default files.)
  • if a non-root user from another group tries to start the process, fail with a proper error saying they do not have permission to do such things as those.