doc: Convert ovs-vlan-test to rST

doc: Convert ovs-vlan-test to rST

Let's start with a simple one that lets us focus on setting up most of
the required "infrastructure" for building man pages using Sphinx.

This changes the 'check-htmldocs' target to 'check-docs' as its now
responsible for building man page docs too.

Other than that, hurrah for (mostly) legible syntaxes.

[1] http://www.tldp.org/HOWTO/Man-Page/q2.html

Signed-off-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Ben Pfaff <blp@ovn.org>