travis: Use container infrastructure.

travis: Use container infrastructure.

Recently some testcases have been failing in travis because of a warning
related to the use of an L3 device (OpenVZ specific) inside the workers.

To get travis tests working again we can move to the newer container
infrastructure: this commit does that.

The disadvantage is that there's no sudo access anymore, but we can
install packages with the apt plugin, and we shouldn't use root for
anything else

Also, since we're building DPDK with vhost-user (not vhost-cuse),
libfuse-dev is not needed anymore.

Tested-at: https://travis-ci.org/ddiproietto/ovs/builds/81764972

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
CC: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>