Enable Updates in installed system (bnc#899999)

*This pull request basically reverts the code removal from commit c9d4f55ce2bcee57f71f9c6593e35d2e92bde432, so the code has been already partly reviewed :wink:.*

### Fix ###

- re-enable the update repositories in the installed system  when updates are disabled during installation (bnc#899999)
- 3.1.127

### Tested in patched GMC installation ###

All works as expected:

- Updates disabled during installation: the `-Updates` repositories were disabled during installation, in the installed system they were correctly enabled
- Updates enabled during installation: the `-Updates` repositories were enabled during installation and as well in the installed system
- AutoYast installation with `<install_updates config:type="boolean">false</install_updates>` config option disabled updates during installation, in the installed system the update repositories were enabled
I think it deserve doc what is in repositories? strings? Repository objects?