Move mirrorbrain to services node

[rgaudin]

Currently mirrobrain (MB) is running on the storage node which implies:

• any load (cpu, mem) to the MB load-balancer affects storage node
• any I/O induced by MB (postgres database queries) affects storage node
• any bandwidth consumed by MB (numerous small queries mostly: redirects to mirrors and MB-specific endpoints like btih, torrent, magnet, metalink)
• should storage node (ie. master mirror) be unavailable for some reason, the whole download.kiwix.org is down while all mirrors are probably still up and ready

It would thus be better for performances and reliability to have the services node host the download.kiwix.org mirrorbrain.

[benoit74]

I think all this comes down to the question of whether it is possible to have mirrorbrain operate without access to "real" files. This has been discussed in the HA discussions, and still needs to be checked AFAIK. If the conclusion is positive, I agree it would be positive to move MB to another node AND have an active or passive backup instance.

[rgaudin]

Sorry I forgot to mention that this ticket is a follow-up of that discussion and it looks like it should be OK although it needs to be tested.