Merge tag 'sparks-2.9.1' into develop

2.9.1
Allow to run a custom installation script which name matches the current sparks role. This is not very elegant but allows *automatic* (eg. automated/industrial) installation of some packages on all machines of a given role.
Add a preinstall target to the custom requirement script.
Create the db_redis() task and upgrade redis-server to the one from Chris Lea's PPA, to benefit of REDIS 2.6.12+ SET(nx=…) argument used in 1flow worker locks.
Use the new task for local REDIS installation.
FIX apt upgrade calls.
bump version in __init__.py
Merge branch 'release/2.9.2'