qa/rgw: allow the rgw teuthology task to capture/set dns names

qa/rgw: allow the rgw teuthology task to capture/set dns names

A teuthology workunit might want to use the rgw task, setting the
rgw-dns-name and/or rgw-dns-s3website-name configuration options to
the fully-qualified domain name. Existing code implies that setting
these configuration options to the empty string will do that. However
the current logic does not support that given it has Python
conditionals that treat the empty string as false. This fixes that.

Now the following teuthology tasks YAML will work as expected:

  tasks:
  - rgw:
      client.0:
        dns-name: ''
        dns-s3website-name: ''

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>