[keycloak-dev] Migration of testsuite's OAuthClient

This PR includes the migration of OAuthClient from the old testsuite to the new arquillian-based testsuite: https://github.com/keycloak/keycloak/pull/2485 There was already another class named OAuthClient in org.keycloak.testsuite.util. So to avoid confusion, this one is named org.keycloak.testsuite.MigratedOAuthClient. Except for the name, MigratedOAuthClient is identical to the one in the old testsuite. So it should work with migrated tests. I've already used it in several of these tests. There is one slight difference in the way it obtains the baseUrl. I have migrated the Constants class and renamed it to MigratedConstants. Instead of the url being hard-coded as in the old testsuite, this url is obtained through a call to AuthServerTestEnricher.getAuthServerContextRoot(). Stan