SCHEDULING-2200 Portal does not call getMyAccountUsage, only getAccountUsage

If the user is the caller, then try to use getMyAccountUsage.

This way a call for a user, authorized to get only his usage, to getAccountUsage
with his username as a parameter will succeed.

It simplifies the portal code to avoid calling two different methods.

Change-Id: I4455e6b7292015163f8641fc8a3e424b6890b333