node: avoid misleading exception in alias property

The Node.alias property was raising an AttributeError in the case that
userdata is None which is masking the real error that the instance has
no alias. Fixed this by using a default value of '' when fetching values
from the user_data dict.
node: run apt-get update before install in apt_install

closes gh-314
awsutils: use instance store bmap for 32bit m1.small

For some unknown 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.

closes gh-329