sshutils: reload scp object if transport is not active

The SCPClient stores a reference to an active transport but does not
attempt to reconnect it if the connection is lost. Updated SSHClient's
'scp' property to also reload the SCPClient in the case that the
transport is not active.

closes gh-119
tests: remove aws_credentials_validation test

This test eventually should be moved to a 'live' tests module that
requires AWS credentials, network connection, and runs using real money
and resources. Removing this test until a 'live' tests module exists
given that all default tests should be purely local and not require an
account or network access.

https://bugs.gentoo.org/show_bug.cgi?id=426528

closes gh-121
config: use chmod 600 when writing config template

Updated exception.ConfigNotFound.create_config to call chmod 600 after
writing the config template. This makes it so that only the owner of the
file can view its contents which is important given that the config
template will eventually contain AWS security credentials.

closes gh-144
cluster: reraise uncaught exceptions in run_plugins

Reraise uncaught exceptions in Cluster.run_plugins instead of logging an
error (along with the fullstack trace) and continuing running other
plugins.
Merge branch 'develop' of git://github.com/jtriley/StarCluster into datacratic_dev