use 64bit AMI as default in config template

Forgot to update the config template a while ago to default to 64bit
AMIs given that they run everywhere now on all instance types.
Discovered this when gh-329 was reported.
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
node: better fix for gh-304

The previous fix simply blasted all /etc/exports entries for each node
before exporting paths to nodes. This can potentially kill other export
paths not being exported by export_fs_to_nodes given that they will not
be redefined. Added `paths` kwarg to Node.stop_exporting_fs_to_nodes
that causes only the specified export paths for each node to be removed
from /etc/exports instead of all entries for each node.

Thanks to @realoptimal for the catch:

https://github.com/jtriley/StarCluster/commit/f7c4967ed891c245cc6d8c64ac38f9fdb345ffd6#commitcomment-4640990
volume: log exception when volume creation fails
volume: only detach if vol != available in _delete_new_volume
cli: raise parser error if tag is incorrectly specified