Fix ironic client ironic_url deprecation warning

In preparation for stein, use 'endpoint' instead of 'ironic_url'
when calling get_client in order to remove the following warning:

WARNING ironicclient.client The argument "ironic_url" passed to
get_client is deprecated and will be removed in Stein release,
please use "endpoint" instead.

For reference:
In the python-ironicclient code, in the ironicclient/client.py#L24
TODO(vdrok): remove in Stein
[...]
  ('ironic_url',): 'endpoint',

Introduced in commit:
https://github.com/openstack/python-ironicclient/commit/58c39b7a80583dd54165cf292ae5dc621e9da361

Change-Id: I1b3ce1955622c40b780c0b15ec7e09be3e8ace72

test-requirements.txt

@@ -11,7 +11,7 @@ mox3>=0.20.0 # Apache-2.0
 psycopg2>=2.6.2 # LGPL/ZPL
 PyMySQL>=0.7.6 # MIT License
 python-barbicanclient>=4.5.2 # Apache-2.0
-python-ironicclient>=2.3.0 # Apache-2.0
+python-ironicclient>=2.4.0 # Apache-2.0
 requests-mock>=1.2.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0