build

Initially, I just wanted to add a new feature to the operator.
This change does much more.
While scanning the code base, I noticed, that editing a secret, for
example adding a new filed to the list of to-be-generated fields, can
lead to the regeneration of every field in the secret.

Also, the error returned by rand.Read wasn't checked.

Furthermore, the client-go used by this project was really old, instead
of just bumping the version, I decided it might be better to just
rewrite this using the operator-sdk.

I also added some integration tests, to make sure the operator behaves
as it's supposed to.