[mongo] Use the primary node only if it belongs to the given replica set

[sgirones]

Connect to nodes that belong to the configured replica set

[calavera]

LGTM

[biilmann]

LVGTM!

[rybit]

lgtm

[sgirones]

Pointing to a node in the replica set:

DEBU[2017-12-14T21:48:11.139687764Z] Dialing database                              component=store-cache hostname=app-web-test-rax-ord-1 replica_set=webpop-production servers="db-rax-ord-1:27017" store=mongo store_id=mongo_new
DEBU[2017-12-14T21:48:11.173144103Z] Got session, Using database bitballoon        component=store-cache db=bitballoon hostname=app-web-test-rax-ord-1 store=mongo store_id=mongo_new

Pointing to a node that doesn't belong to a replica set

DEBU[2017-12-14T21:48:11.216191343Z] Dialing database                              component=store-cache hostname=app-web-test-rax-ord-1 replica_set=webpop-production servers="db-aws-cmh-1:37017" store=mongo store_id=mongo_old
FATA[2017-12-14T21:49:12.033913987Z] Failed to create server                       error="Failed to create store cache: failed to connect to mongo: no reachable servers" hostname=app-web-test-rax-ord-1