Allow enable/disable of kapua-sys user

[gbarbon]

Is your feature request related to a problem? Please describe.
Currently kapua-sys user is enabled by configuration and disabling it is not allowed by business logic.
It would be nice to allow enabling/disabling such user, in order to support the following use case:

• As a platform administrator I want to disable the kapua-sys user after the initial platform configuration has been carried and nominative admin users created so that further administration activities are carried out by the new users.

Describe the solution you'd like
It should be sufficient to remove the check that prevents the update of the kapua-sys user status.

Describe alternatives you've considered
n/a

Additional context
n/a