[AWS|ASG] filtering for ASGs
[AWS|ASG] filter mocked results for describe_auto_scaling_groups

This was causing an issue for me. Because Groups#get uses this method to
fetch a single ASG, it was always returning the first ASG in the data
set, not the one that actually matched the specified ASG name.
Merge pull request #2040 from bgentry/asg-filters

[AWS|ASG] Filters for ASGs.