storage/kubernetes: allow arbitrary client IDs

[ericchiang]

Use a hash algorithm to match client IDs to Kubernetes object names.
Because cryptographic hash algorithms produce sums larger than a
Kubernetes name can fit, a non-cryptographic hash is used instead.
Hash collisions are checked and result in errors.

Closes #631

[ericchiang]

FYI this passes integration testing (since we don't test those on travis)

$ go test -v ./storage/kubernetes/
=== RUN   TestIDToName
--- PASS: TestIDToName (0.00s)
=== RUN   TestNamespaceFromServiceAccountJWT
--- PASS: TestNamespaceFromServiceAccountJWT (0.00s)
=== RUN   TestLoadClient
2016/10/27 16:39:06 third party resource already created "auth-code.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "auth-request.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "o-auth2-client.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "signing-key.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "refresh-token.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "password.oidc.coreos.com"
--- PASS: TestLoadClient (0.11s)
=== RUN   TestURLFor
--- PASS: TestURLFor (0.00s)
=== RUN   TestStorage
2016/10/27 16:39:06 third party resource already created "auth-code.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "auth-request.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "o-auth2-client.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "signing-key.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "refresh-token.oidc.coreos.com"
2016/10/27 16:39:06 third party resource already created "password.oidc.coreos.com"
=== RUN   TestStorage/AuthCodeCRUD
=== RUN   TestStorage/AuthRequestCRUD
=== RUN   TestStorage/ClientCRUD
=== RUN   TestStorage/RefreshTokenCRUD
=== RUN   TestStorage/PasswordCRUD
=== RUN   TestStorage/KeysCRUD
=== RUN   TestStorage/GarbageCollection
=== RUN   TestStorage/ClientConcurrentUpdate
--- PASS: TestStorage (0.55s)
    --- PASS: TestStorage/AuthCodeCRUD (0.07s)
    --- PASS: TestStorage/AuthRequestCRUD (0.05s)
    --- PASS: TestStorage/ClientCRUD (0.09s)
    --- PASS: TestStorage/RefreshTokenCRUD (0.03s)
    --- PASS: TestStorage/PasswordCRUD (0.06s)
    --- PASS: TestStorage/KeysCRUD (0.04s)
    --- PASS: TestStorage/GarbageCollection (0.12s)
    --- PASS: TestStorage/ClientConcurrentUpdate (0.04s)
        transactions.go:48: update1: PUT https://172.15.0.21:443/apis/oidc.coreos.com/v1/namespaces/default/oauth2clients/ojqxonlkoi3gizztonzgm5tign2wczjwnb5hgzdrnxf7fhheqqrcgji Conflict: response from server "{"kind":"Status","apiVersion":"v1","metadata":{},"status":"Failure","message":"Operation cannot be fulfilled on thirdpartyresourcedatas.extensions \"ojqxonlkoi3gizztonzgm5tign2wczjwnb5hgzdrnxf7fhheqqrcgji\": the object has been modified; please apply your changes to the latest version and try again","reason":"Conflict","details":{"name":"ojqxonlkoi3gizztonzgm5tign2wczjwnb5hgzdrnxf7fhheqqrcgji","group":"extensions","kind":"thirdpartyresourcedatas"},"code":409}"
        transactions.go:49: update2: <nil>
PASS
ok      github.com/coreos/dex/storage/kubernetes    0.664s