Copy authConfigs on save so data is not modified

SaveConfig sets the Username and Password to an empty string
on save.  A copy of the authConfigs need to be made so that the
in memory data is not modified.

This is a small issue with the config changes that caused the cli to require the login twice for the push command again after a successful login.

```
docker push crosbymichael/testimage
The push refers to a repository [crosbymichael/testimage] (len: 1)
Processing checksums
Sending image list
Username (): crosbymichael
Password:
Email (): michael@crosbymichael.com
Login Succeeded <<-- successful login but asks to login again
2013/07/25 00:08:56 Please login prior to push. ('docker login')  <<--err
```