Make dependency on netaddr optional

https://review.openstack.org/#/c/38023/ pointed out that
the dependency on netaddr is unnecessary and can be avoided.
Use try_import to avoid a hard dependency on netaddr.

Change-Id: I8b246f20f9001379fcb5ca3a84aabee3847cb945

openstack/common/jsonutils.py

@@ -41,11 +41,12 @@ import json
 import types
 import xmlrpclib
 
-import netaddr
 import six
 
+from openstack.common import importutils
 from openstack.common import timeutils
 
+netaddr = importutils.try_import("netaddr")
 
 _nasty_type_tests = [inspect.ismodule, inspect.isclass, inspect.ismethod,
                      inspect.isfunction, inspect.isgeneratorfunction,
@@ -138,7 +139,7 @@ def to_primitive(value, convert_instances=False, convert_datetime=True,
             # Likely an instance of something. Watch for cycles.
             # Ignore class member vars.
             return recursive(value.__dict__, level=level + 1)
-        elif isinstance(value, netaddr.IPAddress):
+        elif netaddr and isinstance(value, netaddr.IPAddress):
             return six.text_type(value)
         else:
             if any(test(value) for test in _nasty_type_tests):