Merge pull request #36 from lamadon/clean_eip_association

cloud_aws: clean up eip association
cloud_aws: fix string formatting in exceptions
cloud_libvirt: fix string escaping (use r-strings)
general: use new style except syntax for Python 3 compatibility
cloud_aws: add multiple instance types to launch instance

AWS is running out of capacity for certain instance types (e.g. c3.large)
in most regions. Provide the capability to give a list of instance types
to workaround that problem