Add rename instance support to ConfigWriter
Add rename instance support to ConfigWriter

This patch adds support for renaming instances to the ConfigWriter
class. It's needed in here in order to guarantee that we don't lose the
instance during the rename (the instance will be there, either with the
old name or with the new name).

Reviewed-by: imsnah


ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,andir/ganeti,andir/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,yiannist/ganeti,dimara/ganeti,dimara/ganeti,yiannist/ganeti,grnet/snf-ganeti,mbakke/ganeti,andir/ganeti,leshchevds/ganeti,apyrgio/ganeti,grnet/snf-ganeti,leshchevds/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,ganeti/ganeti,mbakke/ganeti,yiannist/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,mbakke/ganeti,onponomarev/ganeti,leshchevds/ganeti