bundleNewAMI should check if necessary Amazon env variables are set before using them.

[andyk]

Amazon environment variables are read in by and set as variables in the AWSConnection trait in aws.scala.

If the enviornment variables are not set when bundleNewAMI is called the function should fail with a useful message telling the user which environment variable they need to set.

These are the relevant lines in bundleNewAMI()

this.executeCommand("mv ~/.tags /tmp/mesos-ec2-tags")
this ! "ec2-bundle-vol -c /tmp/%s -k /tmp/%s -u %s --arch %s".format(ec2Cert.getName, ec2PrivateKey.getName, userID, "x86_64")
this ! "ec2-upload-bundle -b %s -m %s -a %s -s %s".format(bucketName, "/tmp/image.manifest.xml", accessKeyId, secretAccessKey)