Expand JJB macros in conditional-step

Cherry-picked from:
https://review.openstack.org/#/c/471030/1

When the conditional-step builder is passed a single JJB macro that
expands to several builders, only the last one will make it in the job
configuration.

The steps seems to now be handled as regulard build steps, probably
since 1.3.2 and
https://github.com/jenkinsci/conditional-buildstep-plugin/commit/066515eab1dc35aa8fa9073cbfc71744fe8eb30d

tox -e venv -- testr run tests.macros

Change-Id: I9684acf404ff537fc380374224238eea5f20faa5