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 git@fabrykowski.pl

[torgiren]

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