Allow describe_instances to use tags for searches

In the EC2 API, the DescribeInstances API call allows for a 'filter'
argument to be passed, consisting of key/value filter pairs.  If the
filter contains tag:<something> or tag-<something>, the search is done
on resource tags.

See
http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeInstances.html
for details of the API and

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Using_Tags.html for
usage.

Change-Id: If2622c4dacbdd3a53fd6d49242225692da8b741a
Signed-off-by: Stephen Gran <stephen.gran@guardian.co.uk>