Make EphemeralLvm::Helper support all EC2 instance types

This helper module is broken and raises an exception when executing against the **m1.small** and **c1.medium** instance types in AWS.  This happens because the block device for those instances is named */dev/sda2* ([see here](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html#InstanceStoreDeviceNames)) and the current logic does not allow for a numeric suffix.

This PR adds a test to verify the failure and extends the helper to allow for a single digit at the end of the device name.