awsutils: omit root snapshot id in custom block dev map

It's unclear whether this never worked or recently stopped working given
that users at one point in time were able to launch clusters with 0.94.2
and suddenly can't now. Either way, removing the snapshot id from the
root device's custom block device mapping when setting
delete_on_terminate still works and still inherits the AMI's underlying
root device snapshot anyway.

closes gh-330
use 64bit AMI as default in config template

Update the default config template to use 64bit AMIs given that they run
everywhere now on all instance types.
awsutils: use instance store bmap for 32bit m1.small

For some reason the only way to get StarCluster's custom block device
mappings for ephemeral drives to work with m1.small and a 32bit EBS AMI
is to use instance_store=True when calling create_block_device_map.
docs: fix sphinx warnings from Changelog
bump version to 0.94.3
Changelog: add updates for 0.94.3 release
Merge branch 'hotfix-0.94.3'