(#11767) No longer necessary to delete ssl directory in each test

Previously, each TestCase that used with_master_running_on had to
remember to delete the ssl directories from every host. Otherwise, if
either the master or any of its agents used a certificate from a
previous run, the test would fail.

Also, previously, TestCases were not consistent in how they referenced
the ssl directory: /etc/puppet/ssl, #{host['puppetpath']}/ssl,
on the configuration file being used.

Deletion of the ssl directories now happens within the
with_master_running_on method itself, so that individual TestCases
don't need to.