ceph_migrate: rewrite a variable to avoid linting issues

This change does not change the behavior of the code, but
it works with newer ansible-lint. The version of ansible-lint was
ancient and now it needs to be updated due to some changes in ansible
itself which led to:
ModuleNotFoundError: No module named 'ansible.parsing.yaml.constructor'

The update led to a new error when parsing the definition of the pct
variable, namely:
tests/roles/ceph_migrate/tasks/ceph_validate.yaml:42:10:
  jinja[invalid][/]: Error rendering template: float division by zero

Actual code change by Francesco Pantano.
Commit message by Luigi Toscano.

Author: fmount

tests/roles/ceph_migrate/tasks/ceph_validate.yaml

@@ -39,7 +39,10 @@
   vars:
     num_up_osds: "{{ ceph | community.general.json_query('osdmap.num_up_osds') | default(0) }}"
     num_osds: "{{ ceph | community.general.json_query('osdmap.num_osds') | default(1) }}"
-    pct: "{{ (num_up_osds | float | default(0.0) / num_osds | float | default(1.0)) * 100 }}"
+    pct: >-
+      {%- set up_osds = (num_up_osds | float | default(0.0)) -%}
+      {%- set total_osds = (num_osds | float | default(1.0)) -%}
+      {{ (up_osds / total_osds * 100) if total_osds > 0 else 0 }}
   block:
     - name: OSDMap
       ansible.builtin.debug: