[api] Require running memcached.

Fixes: https://github.com/openSUSE/open-build-service/issues/454
[webui] port to possibly_empty_ul

This avoids duplicating conditions
[api] rename bs_user_id to user_id in all tables
[api] put the user login in the fixtures instead of the id

Same for roles
[dist] update spec file template to buildrequire procps