reana-admin: add retention-rules-apply command

[mdonadoni]

Add command to apply pending retention rules, deleting the necessary files or directories.

How to test:

1. Modify one of the demo examples, defining some retention rules
2. Fake a different current time (e.g. one day in the future)

--- a/reana_server/reana_admin/cli.py
+++ b/reana_server/reana_admin/cli.py
@@ -748,6 +748,7 @@ def check_workflows(
 def retention_rules_apply() -> None:
     """Apply pending retentions rules."""
     current_time = datetime.datetime.now()
+    current_time = current_time + datetime.timedelta(days=1)
     pending_rules = WorkspaceRetentionRule.query.filter(
         WorkspaceRetentionRule.status == WorkspaceRetentionRuleStatus.active,
         WorkspaceRetentionRule.apply_on < current_time,

3. Run the retention-rules-apply command

kubectl exec -it deployments/reana-server -- flask reana-admin retention-rules-apply

Expected result: The retention rules are applied correctly and the correct files are deleted.

[codecov-commenter]

Codecov Report

Merging #525 (3e3acdb) into master (1ac23c4) will increase coverage by 0.70%.
The diff coverage is 90.56%.

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   52.62%   53.32%   +0.70%     
==========================================
  Files          29       30       +1     
  Lines        2805     2858      +53     
==========================================
+ Hits         1476     1524      +48     
- Misses       1329     1334       +5     

Impacted Files	Coverage Δ
reana_server/reana_admin/retention_rule_deleter.py	90.47% <90.47%> (ø)
reana_server/reana_admin/cli.py	48.52% <90.90%> (+1.41%)	⬆️

[VMois]

Very nice job!

One small thing to do is to add a line to CHANGES.

[VMois]

Nice job!