Complement set Metadata server tests

[Mauro S M Rodrigues]

There is no set Metadata test to check when the method raises HTTPRequestEntityTooLarge
It is raise in https://github.com/openstack/nova/blob/master/nova/api/openstack/compute/server_metadata.py#L136

[Giampaolo Lauria (lauria)]

I think that the tests already exist.
Please refer to:
 def test_invalid_metadata_items_on_create(self):
in:
 /opt/stack/nova/nova/tests/api/openstack/compute/test_server_metadata.py
line:
430.

This method tests 3 different scenarioes of invalid metadata items.