AddUserSecrets() should use configurationBuilder.GetFileProvider() to read from project.json

[divega]

Based on the discussion with @DamianEdwards @glennc @davidfowl and others today, the UserSecrets configuration provider should read project.json from the same location used for configuration files, i.e. it should use the IFileProvider obtained from configurationBuilder.GetFileProvider() to read the file.

In addition, if the project.json file is not found it shouldn't fail.

@muratg we need someone to make this change for RC2.

[muratg]

@BrennanConroy can you take this?

I assume this still won't solve #62...

[divega]

@BrennanConroy I already asked @smitpatel to fix aspnet/Configuration#422. Once we have configurationBuilder.GetBasePath() you should use in this fix. Please coordinate with @smitpatel in case one of you wants to take over the whole fix.

[divega]

Actually scratch most of that. UserSecrets should not call builder.GetBasePath() but builder.GetFileProvider() to obtain a file provider to gain access to the project.json file. I will update the description of the bug.

[BrennanConroy]

Fixed via 6cc0039