ceph-ansible-nightly: fix find for latest stable tag

ceph-ansible-nightly: fix find for latest stable tag

Docker Hub API when queried only returns the 10 first tags. This is
annoying but we have to:

* count the number of pages
* query all the tag from each page
* build a variable with all the tags
* transform this variable in an array
* sort the array
* pick the last element of the array which corresponds to the latest
stable tag

I could have done that with a loop (for luminous and jewel) but assigned
variable in array's name is a nigthmare in bash so I ended up doing
twice the same thing...

This is not elegant but it works.

Signed-off-by: Sébastien Han <seb@redhat.com>