Require including prefix in application name

Previously, when searching for instances in autoscaling groups with application `foo`, environment `bar` and group `workers` would return results from groups with names like

`foo-bar-X123-workers-Z456`

and

`widget-foo-bar-X789-workers-A321`

This could potentially be confusing when results from the two groups should be treated entirely different.

When the CloudFormation stack has a prefix in the name like the second example above, it is now mandatory to include such a prefix in the application name when searching for autoscaling instances or when using `mulder` with Capistrano.

For a CloudFormation stack for application `fop-foo` with environment `bar` and an AutoScaling group with `webservers` in the name:

```ruby
set :aws_creds,		'config/aws.yml'
set :stack_name,	'fop-foo'
set :rails_env,		'bar'
set(:mulder)  		{ Mulder::Capistrano.new(aws_creds, stack_name, rails_env) }

role(:web)    { mulder.ips('webservers', true) }
```

```
$ mulder search fop-foo bar webservers
```

Both will return instances from an autoscaling group called `fop-foo-bar-X123-webservers-Y456`, but not from the group `foo-bar-X123-webservers-Y456`.