add support for customizable username field

[torgiren]

Problem to solve: I have an ldap backend. dex returns an user with:

{
...
name: user1
email: some-email@some-domain
...
}

dex-k8s-authenticator creates a kubeconfig contains: some-email-<cluster-name> and I would like to have: user1-<cluster-name>

Signed-off-by: Marcin Fabrykowski [email protected]

[torgiren]

any chance to review this? It looks like all PRs failes e2e tests :(