Migrate to Go modules

[andreyvelich]

We should migrate to Go modules from dep since it is not longer supported from 2020: https://github.com/golang/dep#dep.
Also to avoid errors like this: #1436 (comment).
Currently, I am using the same package versions with k8s.io/kubernetes v1.13.3 and sigs.k8s.io/controller-runtime v0.1.9 which is very old.

Unfortunately, I can't switch to the newer version because of this: #1405.
Controller runtime doesn't have dynamic webhook creation from v0.2.0.
We should disable dynamic webhook creation, update the versions and clean-up modules.

As well I fixed folder name in Build Docker images GitHub actions workflow. What do you think @DomFleischmann @RFMVasconcelos ?

Please take a look @gaocegege @johnugeorge.

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[andreyvelich]

/retest

[andreyvelich]

The tests are passed.
Can you check it @gaocegege @johnugeorge, please ?

[johnugeorge]

/lgtm

[andreyvelich]

/retest

[gaocegege]

Should we use K8s v1.16.9 like tf-operator https://github.com/kubeflow/tf-operator/blob/master/go.mod#L33

[andreyvelich]

Should we use K8s v1.16.9 like tf-operator https://github.com/kubeflow/tf-operator/blob/master/go.mod#L33

I think once we fix problems with Webhooks we can directly switch to the latest version of controller-runtime - v0.8.2 and K8s. - v1.19 or v1.20