The original patch assumes that both --external and --is-default are set
in the same request and broke case when --is-default is set as an
network update. The validation logic have to be moved on API side to
avoid extra API calls from openstackclient.
This reverts commit 962efd949feb461283a9bb4a668fbd310f80ba40.
Related-Bug: #1745658
Change-Id: Idf08abb0e08a6880f89c3e9df9dd2ac82f36c432