Update relevant artifacts to add support for DPDK 16.04.

Update relevant artifacts to add support for DPDK 16.04.

Following changes are applied:
 - INSTALL.DPDK.md: CONFIG_RTE_BUILD_COMBINE_LIBS step has been
   removed because it is no longer present in DPDK configuration
   (combined library is created by default),
 - INSTALL.DPDK.md: VHost Cuse configuration is updated,
 - netdev-dpdk.c: Link speed definition is changed in DPDK and
   netdev_dpdk_get_features is updated accordingly,
 - netdev-dpdk.c: TSO and checksum offload has been disabled for
   vhostuser device.
 - .travis/linux-build.sh: DPDK version is updated and legacy
   flags have been removed in configuration.

Signed-off-by: Michal Weglicki <michalx.weglicki@intel.com>
Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>