ceph-volume: add --all flag to simple activate

ceph-volume: add --all flag to simple activate

This is intended to behave similarly to the lvm activate --all argument.
When passed, c-v will scan /etc/ceph/osd/ (or the location specified by
CEPH_VOLUME_SIMPLE_JSON_DIR) for json files (glob *.json) and call activate
for each file.  This should greatly ease the take-over of ceph-disk OSDs
with manual commands and deployment tools like DeepSea and ceph-ansible.
Also adds a simple unit test and modifies the
simple/centos7/filestore/activate functional test to use --all.

Signed-off-by: Jan Fajerski <jfajerski@suse.com>