The is_device_mounted has incorrect assumptions about disk naming

[Peter Sabaini]

The utility function charmhelpers.contrib.storage.linux.util.is_device_mounted() assumes that device names that end in numbers are partitions. This assumption is overly broad, it doesn't hold in general eg. for loop devices, device names that are passed in via physical path (/dev/disk/by-path) and device names that are passed in by-label or by-uuid if those happen to end in numbers.

The result is that the function will return False even if a partition on the passed-in device would be mounted. This could potentially be quite dangerous.