Skip test_impala_ldap_connect_user_agent in case of DEFAULT_AUTH (#501)
Skip test_impala_ldap_connect_user_agent in case of DEFAULT_AUTH (#501)

The test above was added in #498 and does not work with a default
Impala cluster.

This is a test only change to make tests greens when using
default authentication (=no authentication).
cloudera/impyla,cloudera/impyla