#697 add capability to expend env variable in username

[QuentinBrun]

I hope this help.

[JoC0de]

@QuentinBrun thanks for the contribution.
I am not sure if we can implement it in this easy way. The UserName property should be renamed to SavedUserName to be consistent with the SavedPassword property. But it affects the public interface INugetPackageSource witch impacts our plugin system.
@igor84 what is your opinion on this?

[igor84]

Our plugin system is still very young so I would be surprised if anyone beside me is using it :) and since I don't use username in my plugin and configuration it wouldn't really break anything... So I would be ok with changing it.

[QuentinBrun]

Hi, thank you!