Configure instance VPC with GROUP and SUBNET vars.

Previously, pave could not create instances running on subnets or set
security groups. See .env.sample for details of how to use these
variables.
Merge pull request #3 from MarkBennett/add-support-for-groups-and-subnets

Configure instance VPC with GROUP and SUBNET vars.
Handle times when the instance name cannot be set.

This was occurring when the instance id was reported as invalid by the
EC2 client. We're assuming this is a timing issue as it's a new image.
Merge pull request #5 from MarkBennett/handle-invalid-id-when-naming-instance

Handle times when the instance name cannot be set.
Assume staging environment if not specifiied.

This means that an environment will always be set.
Merge pull request #6 from MarkBennett/default-environment-to-staging

Assume staging environment if not specifiied.