ceph-volume: fix blkid on inaccessible devices

ceph-volume: fix blkid on inaccessible devices

There exists block devices which can not be accessed. For example DRBD
/dev/drbdX in secondary state. `blkid` can not read from this:

  $ blkid -c /dev/null -p /dev/drbd4
  blkid: error: /dev/drbd4: Wrong medium type

Since release 17.2.7 this results in "CEPHADM_REFRESH_FAILED: failed to
probe daemons or devices"[1] and the following trace:

  File "/usr/lib/python3.6/site-packages/ceph_volume/util/device.py", line 482, in is_partition
    return self.blkid_api['TYPE'] == 'part'

Fix this by using `get()` method, so that it defaults to `None` if the
key does not exist. And while at it also switch to `get()` in an other
reference to this key.

Probably caused by commit 2422ad867dff9d526d7e8be543178c897991097f ???

[1] https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/ZSD4OPDMCEXOW74IJH4L4D5PQXFFINGS/

Signed-off-by: Sascha Lucas <sascha_lucas@web.de>