ci: add missing dependencies for documentation

ci: add missing dependencies for documentation

Install missing dependencies so that doc can be generated.
While at it, explicitly configure that we want the doc to be generated.
Missing dependencies are then reported as an error rather than silently
ignored.

Because of these extra dependencies, only build them in dedicated travis
jobs.

Fixes: ad2b2cfb1ea3 ("ci: enable unit tests with Travis")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>