Was able to reset password for the non-administrator users. But not for the users with administrator directory roles.
Tried as documented: https://msdn.microsoft.com/Library/Azure/Ad/Graph/api/users-operations#ResetUserPassword
Response: Code: Authorization_RequestDenied Message: Insufficient privileges to complete the operation.
I configured the application in AzureAD with all application and delegated permissions (Read and Write Directory Data, etc).
Googled and found one related post: the permission scope was changed recently http://blogs.msdn.com/b/aadgraphteam/archive/2015/10/06/new-graph-api-consent-permissions.aspx