Bluetooth: tests/shell: Implement "clear" command

Implement a new "clear" command to exercise the bt_storage API. The
command can either take an address parameter to clear storage for a
specific device, or, if called with the parameter "all", to clear the
entire storage.

Jira: ZEP-181

Change-Id: I1ebd17033f2062951abeeb7d7c448a34a3672577
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>