EC2: do not assume order in dictionary

Instance tags dictionary used to be empty, so to assert addition
of a new tag, fetching the first entry from that dict worked fine.

But it doesn't work when there are more than one entries are
present. This patch fixes this.

Change-Id: I8130d16b6d30498f77dc23368ab4b5a9eff60b7c
Merge "EC2: do not assume order in dictionary"