Make role deletion robust towards interrupted creation

If role creation gets interrupted along the way, we could end up with
either a detached IAM role or a detached IAM instance-profile.

Don't let that hinder deletion, but also don't try to be any smarter: if
the names don't match, it's not an hc2002 role and the function will
fail without deleting role or instance profile.
Add support for VPC connections
Generalize AMI search plugin to cover other resources

The new 'search' plugin will query AWS for security-groups, snapshots
and subnets, in addition to images.
Introduce vpc plugin

The new plugin builds on top of the 'search' plugin, enabling
DescribeVpcs queries to specify a VPC, which then is used to restrict
security-group and subnet queries.

Security-groups for instances launched in a VPC can now be specified by
name as well.
Accept AWS_DEFAULT_REGION environment variable

The variable is recognized by the AWS CLI tool, as well.