api-ref: Add a description of 'key_name' in rebuild
api-ref: Add a description of 'key_name' in rebuild

In rebuild operation, users can unset the existing keypair
by setting 'key_name' to 'null' in a request body.
(The function has been added in
I23886a89c25f811cfbe7e2500ce7ff52f9162966).

But it is not described obviously in API reference.
So add an additional description in API reference.

Change-Id: I2d1b8eddd8555ad6896541b308895dc537686168

klmitch/nova,mahak/nova,phenoxim/nova,mikalstill/nova,gooddata/openstack-nova,rahulunair/nova,gooddata/openstack-nova,klmitch/nova,mahak/nova,openstack/nova,gooddata/openstack-nova,rahulunair/nova,rahulunair/nova,klmitch/nova,openstack/nova,openstack/nova,gooddata/openstack-nova,phenoxim/nova,mahak/nova,mikalstill/nova,mikalstill/nova,klmitch/nova