awsutils: fix custom block dev map for S3 instances/AMIs

The custom runtime block device map generation feature implemented in
0.94 doesn't work with instance-store (ie S3-backed) AMIs and gives an
error about invalid block device maps when attempting to launch
instances.

Added instance_store kwarg to EasyEC2.create_block_map that generates a
valid custom block device mapping that works for S3-backed
AMIs/instances. Updated EasyEC2.request_instances to use this new
instance_store kwarg in the case of S3-backed AMIs.

closes ContinuumIO/anaconda-ec2#7
s3image: fix bug when host instance is EBS-backed

Updated S3ImageCreator to explicitly specify the block device mapping
rather than inheriting it from the host instance. This fixes an issue
where trying to create an S3 image from an EBS-backed instance fails due
to the EBS host's block device mapping not being valid for S3 AMIs.  We
avoid this by explicitly generating an S3-compatible block device
mapping that enables all ephemeral drives on any instance and passing
this to the ec2-bundle-vol command.

closes gh-300