heat tests : add coverage for instance UpdateReplace

Add some tests which test the UpdateReplace path for Instance

Change-Id: Ib9c35498c3ab2811fcd73aac8159315cb6251004
Instance resource remove unreachable/redundant Metadata update check

update_template_diff already throws resource.UpdateReplace if the
json_snippet contains changes other than to Metadata, so the else
here is unreachable and redundant

Change-Id: I9f0ad88d0b6fa65cafd5b2843a38913fa843a3ac