Lisk-sdk: Set default db user to 'lisk'

[Nazgolze]

lisk-scripts in 1.1 and later sets DB_USER based solely on what's in the config

[Tschakki]

These changes require the README.md to be updated.
Please change the prerequisites like in https://lisk.io/documentation/lisk-core/setup/pre-install/source

We could also think of adding a troubleshooting case and different solutions for error: role "lisk" does not exist to the docs, e.g. coldstart or changing db username manually in config

[diego-G]

Thanks @Tschakki. Could you please open a PR against 1.1.0 to update the README?

[diego-G]

Readme needs to be updated.