get instance object from compute api
get instance object from compute api

Fix instance has no attribute 'info_cache' issue when
associate/disassociate floating ip.

Fix redmine #10873

Signed-off-by: blkart <1085ae7ac5d420d52a04a26b37297224b6df9d8b@gmail.com>

eayunstack/nova,eayunstack/nova