Reduce nuget output

This addresses part of #1623. Unfortunately, because the CLI takes Nuget
as a binary, it is hard to get to where I think we should really be.
This change makes default verbosity "minimal", which is the first level
where you get any status output. Unfortunately, things like package
downgrade warnings and the like still appear there. This does hide all
the "info" and "trace" messages by default.

I also removed the now useless (and previously undocumented)
--quiet.

test/Performance/HelloWorld.cs

@@ -81,7 +81,7 @@ namespace Microsoft.DotNet.Tests.Performance
 
             var restoreCommand = new RestoreCommand();
             restoreCommand.WorkingDirectory = RestoredTestProjectDirectory;
-            restoreCommand.Execute("--quiet").Should().Pass();
+            restoreCommand.Execute().Should().Pass();
         }
     }
 }