Use namespace names in NetcatTester

Use namespace names in NetcatTester

Currently NetcatTester requests namespace IPWrapper instances as
client/server_namespace arguments but functional tests commonly use
namespace names as arguments not IPWrapper instances (because IPWrapper
is cheap to instantiate) and NetcatTester needs only namespace names.

This change requests names as NetcatTester client/server_namespace
arguments in order to simplify NetcatTester and its use.

Change-Id: Ic4a297efdeaef00e70892d1c871ce1c9174055b8