I think there is no default user. You can define your a user in manager-gui
role to be able to use it with /manager/html
this definition goes into wwwroot\bin\apache-tomcat-7.0.52\conf\tomcat-users.xml
You can edit that file in FTP and add your user, but an easier way would be to go into https://<YourSiteName>.scm.azurewebsites.net/DebugConsole
and log-in with your deployment credentials azure account and go to that file and edit it in the browser. You can also use Monoco (Visual Studio Online) https://<YourSiteName>.scm.azurewebsites.net/Dev
but you need to enable it in portal first, then use your deployment credentials again.
After you get to the XML, you need to add something like the following to your <tomcat-users>
tag
<role rolename="manager-gui"/>
<user username="admin" password="admin" roles="manager-gui"/>
Please restart your website after the changes takes place.