Replica sets re-enabled for local provider

[Michael Foord]

Hey all,

When we switched to replica sets we explicitly disabled them for the 
local provider because it caused mongo to fail to start for "some 
people". It turns out that along the path of "enabling write-majority 
for mongo" it would be convenient if the local provider was less of a 
special snowflake and also used replica sets. It also turns out that the 
problem with mongo not starting *appears* to have gone away.

We've now re-enabled the use of replica sets for the local provider [1]. 
Please try it out, especially if you had difficulties with this before, 
and report back if there are any problems.

All the best,

Michael Foord

[1] https://code.launchpad.net/~mfoord/juju-core/local-replset/+merge/221117