Pin docutils to 0.9.1

Docutils 0.10 breaks manpage generation in sphinx 1.1.3. A fix to
sphinx has been merged, but not yet released (scheduled for dec
24-31). In the mean time, we should pin docutils to 0.9.1.

Similar change was present before as
I157f3c191bfc09d2191a3508d81bcbaea56205a6 but was incorrectly removed by
later requirements syncs.

Closes-Bug: 1091333
Change-Id: I157f3c191bfc09d2191a3508d81bcbaea56205a6