Fix sphinx 'manpage' role
Fix sphinx 'manpage' role

Building the documentation with manpages fails with sphinx >= 1.6
because of an internal change [1]. By explicitly setting the domain to
'None' when translating the 'manpage' node into a 'doc' node, sphinx's own
transformation to handle this change is triggered.

[1] https://github.com/sphinx-doc/sphinx/issues/3698

ganeti/ganeti,ganeti/ganeti,ganeti/ganeti