configure/packaging: introduce Cython dependency

configure/packaging: introduce Cython dependency

To prepare for the migration of rbd.py to Cython, add Cython as a package
dependency and have the configure script look for it.

Also adds Cython as a dependency for admin/build-doc, and changes the
virtualenv to allow usage of system packages (to avoid having to compile
Cython inside the virtualenv).

With contributions by Josh Durgin <jdurgin@redhat.com>

Signed-off-by: Hector Martin <marcan@marcan.st>