Further harden boto version checking in EC2 tests

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

Fixes Bug #1239220

Change-Id: I455ce5f2aabfad7178be361844c6b9b7b0d9c946
Merge branch 'stable/grizzly' of https://github.com/openstack/nova into grizzly/stable