api-ref: document user_data length restriction
api-ref: document user_data length restriction

user_data for server create and rebuild is limited to
65535 bytes in the schema, so we should be clear about
this limit in the API reference.

Change-Id: I61fe749ef20229744e1420779ae8128192704554

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