Be able to define the search scope

[pjchartre]

Just parametrizing the SearchControls used in com.yammer.dropwizard.authenticator.LdapAuthenticator

        SearchControls searchControls = new SearchControls();
        searchControls.setSearchScope(SearchControls.SUBTREE_SCOPE);

Bad luck, I cannot override easily the class com.yammer.dropwizard.authenticator.LdapAuthenticator because all methods are private :( I had to clone quite all of this class...