Increase socket timeout to 1 minute during bootstrap

We increase the socket timeout during bootstrap to
cope with slow disk I/O during machine initialisation.
Empirical evidence has shown this to decrease the
bootstrap failure rate on Microsoft Azure.

Fixes https://bugs.launchpad.net/juju-core/+bug/1351101
So, the remaining places that use mgo.DialWithInfo directly are...
worker/peergrouper/initiate.go (could probably switch?)
worker/singular/mongo_test.go (harmless?)
mongo/admin_test.go (harmless)