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
Not worth the trouble at the moment. peergrouper threads mgo.DialInfo through param structs, and the others are just tests. I'll tidy it up when forward porting.