Rename ext_network to floating_network in Rally tasks

Rename ext_network to floating_network in Rally tasks

It now fails [1]:
Traceback (most recent call last):
  File "/src/rally/rally/task/runner.py", line 70, in _run_scenario_once
    getattr(scenario_inst, method_name)(**scenario_kwargs)
  File "/usr/lib/python3.8/site-packages/rally_openstack/task/scenarios/nova/servers.py", line 1115, in run
    floatingip = self.neutron.create_floatingip(**create_floating_ip_args)
  File "/src/rally/rally/task/service.py", line 114, in wrapper
    return func(instance, *args, **kwargs)
  File "/src/rally/rally/task/atomic.py", line 91, in func_atomic_actions
    f = func(self, *args, **kwargs)
TypeError: create_floatingip() got an unexpected keyword argument 'ext_network'

[1] http://artifacts.opnfv.org/functest/JOGVTC31O1IL/functest-opnfv-functest-smoke-latest-rally_sanity-run-905/rally_sanity/rally_sanity.html#/NovaServers.boot_server_associate_and_dissociate_floating_ip/failures

Change-Id: I7157847980b417b814d15e4bf3bbac3c4f114690
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>