doc: Don't override default theme

doc: Don't override default theme

Sphinx 1.3 renamed the 'default' theme to 'classic' and configured the
'alabaster' theme as the new default. To prevent breaking existing
builds, the 'default' name was reserved as an alias for 'classic' [1].
However, initially this raised a warning [1] with a message to use
'classic' instead. This warning was removed in 1.3.2 [2], but it will
result in errors (due to the use of the '-W' flag) for Sphinx 1.3.0 and
1.3.1 users.

Mitigate the issue by not setting a theme if the 'ovs_sphinx_theme'
package is absent. This will result in Sphinx using its default theme,
be that 'classic' (Sphinx < 1.3) or 'alabaster'.

[1] https://github.com/sphinx-doc/sphinx/commit/68021b0bd
[2] https://github.com/sphinx-doc/sphinx/commit/034c4e942

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