Fixed Readme and ceph-disk activate if statement

[akadoya]

1. Readme says 'device' is the key for OSD data device but codes are actually using 'data'.
2. bash testing operator should be executed inside of bash resource rather than execute. (otherwise, this test case fails with [[ not found in some platforms)
3. is_device is misleading name but it actually checks if it's a partition as mentioned in the comments. so ceph-disk activate should be executed with #{partitions} when is_device is empty = it's not including a partition number.
   Also added 2 lines to make debugging easier from chef client log.

[akadoya]

Any update on this? I am adding some changes so this cookbook works in the real life in my fork. I'd love to contribute to this upstream, too.