[keycloak-dev] Migration of testsuite's OAuthClient

[Stan Silvert]

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