Use cron in pollscm

Fixes this warning

WARNING:jenkins_jobs.modules.triggers:Your pollscm usage is deprecated,
please use the syntax described in the documentation instead

Also it is recommended by the docs
https://docs.openstack.org/infra/jenkins-job-builder/triggers.html#cron

And is done every where else in ci
https://github.com/GerritCodeReview/gerrit-ci-scripts/search?utf8=✓&q=pollscm+&type=

Change-Id: I534469e2d54da7f2b4908dbc6cc87d2ee4d717c1
1 file changed
tree: 48d4f8b8313d6b4a04e3d57e1e6adeaa83980b0d
  1. jenkins/
  2. jenkins-docker/
  3. .gitignore