Further harden boto version checking in EC2 tests

Use pkg_resources to determine correct boto interface
to use in API tests.  stable/havana version.

Fixes Bug #1239220

Rebased-By: Dirk Mueller <dirk@dmllr.de>

Change-Id: I455ce5f2aabfad7178be361844c6b9b7b0d9c946
Merge "Further harden boto version checking in EC2 tests" into stable/havana