Fix the issue to unescape unneed characters in image property

Currently, powervc driver image sync service will sync the powervc image
meta properties to hosting openstack side, it's using
HTMLParser.unescape() to unescape the escape chars. It's over used, only
"<", and ">" that corresponding escpae char is "&lt;" & "&gt;" is needed
to be unescaped. Fix this only to unescape this two chars.

Closes-Bug: #1362886
Change-Id: I11db0cab46c8707a6c306f9967d7afc08fbad3ec