services: Add Gitolite.

* gnu/services/version-control.scm (<gitolite-configuration>,
<gitolite-rc-file>): New record types.
(gitolite-accounts, gitolite-activation): New procedures.
(gitolite-service-type): New variables.
* gnu/tests/version-control.scm (%gitolite-test-admin-keypair, %gitolite-os,
%test-gitolite): New variables.
(run-gitolite-test): New procedure.
* doc/guix.texi (Version Control): Document the gitolite service.