mongo: move MongoInfo type

This PR moves the `MongoInfo` type from the `environmentserver/authentication` package to the `mongo` package.

This change is necessary to unblock a larger change which was hitting a circular dependency.

With that said, as this change consists of _removing_ the import of the `environmentserver/authentication` from most packages (and renaming the type), it seems clear in hindsight that the `MongoInfo` type belongs in the `mongo` package.
On 01/09/14 20:30, Dave Cheney wrote:
> In environmentserver/authentication/authentication.go:
> 
>> @@ -14,20 +14,6 @@ import (
>>  	apiprovisioner "github.com/juju/juju/state/api/provisioner"
>>  )
>>  
>> -// MongoInfo encapsulates information about cluster of
>> -// servers holding juju state and can be used to make a
>> -// connection to that cluster.
>> -type MongoInfo struct {
> 
> Also, MongoInfo embeds a mongo.Info, so renaming the outer type doesn't
> solve the problem of `mongo.MongoInfo.Info`. O-O

Fair enough. LGTM