The veth is moved from the container NS to the host NS.
This is handled by the code that sets the link to UP but the wrong
hostVeth is returned to the calling code.
If interface name for a container provided by a user is already present,
Veth creation fails with incorrect error.
If os.IsExist error is returned by makeVethPair:
* Check for peer name, if exists generate another random peer name,
* else, IsExist error is due to container interface present, return error.
Fixes #155
