Ec2Signer : Allow signature verification for older boto versions

Since the fix for bug #1197553, verification for older clients (which strip the
port when formatting the request) fails.

This conditionally reverts to the original behavior, by detecting the boto
version via the User-Agent header, the default behavior will be the new
behavior (which doesn't strip the port), but this will allow a less painful
transition for clients/distros to the new boto version.

Fixes bug #1205281

Change-Id: I54ac9c5ba91e697004f1346a8f2d685da488992a