Changing default polling interval from 60 to 30 (#621)

Proposing this change following email discussion on making it less conservative. What do you think of this change?

Now it makes me think, can we update the `DefaultRetryDuration` from 30 seconds to 15 seconds if the change above is fine?

Co-authored-by: Joel Hendrix <jhendrix@microsoft.com>

autorest/client.go

@@ -31,7 +31,7 @@ import (
 
 const (
 	// DefaultPollingDelay is a reasonable delay between polling requests.
-	DefaultPollingDelay = 60 * time.Second
+	DefaultPollingDelay = 30 * time.Second
 
 	// DefaultPollingDuration is a reasonable total polling duration.
 	DefaultPollingDuration = 15 * time.Minute