Pluggable provider for instance types metadata

[joesondow]

For the case of Eucalyptus, the Amazon instance types metadata needs to be configurable. The definitions of the EC2 instance types are different in Eucalyptus, so getting the metadata from Amazon's published information doesn't make sense. Allow the Asgard admin to override the source of data for instance types metadata with their own implementation.

Needed by @grze for Eucalyptus support