openstack-dashboard depends "apache2 | httpd" but uses a script on postinst not provided by all providers of httpd

In the postinst script for openstack-dashboard, the script "/usr/share/apache2/apache2-maintscript-helper" is sourced with no checks to confirm it is there, resulting in an installation failure. I am using nginx as my web server, which the current requirements of "apache2 | httpd" for openstack-dashboard satisfies.

I don't know for sure whether updating the depends to remove httpd or having the postinst script test for the existence of /usr/share/apache2/apache2-maintscript-helper is the correct course of action. I'd prefer a test and printed warning if unable to enable a config for apache2.

The error:
  /var/lib/dpkg/info/openstack-dashboard.postinst: 42: .: Can't open /usr/share/apache2/apache2-maintscript-helper

The relevant code:
  # apache 2.4
  . /usr/share/apache2/apache2-maintscript-helper
  apache2_invoke enconf $CONF || exit $?

Description: Ubuntu 16.04.3 LTS
Release: 16.04
$ apt-cache policy openstack-dashboard
openstack-dashboard:
  Installed: 3:12.0.1-0ubuntu1~cloud0
  Candidate: 3:12.0.1-0ubuntu1~cloud0
  Version table:
 *** 3:12.0.1-0ubuntu1~cloud0 500
        500 http://ppa.launchpad.net/ubuntu-cloud-archive/pike-staging/ubuntu xenial/main amd64 Packages
        500 http://ppa.launchpad.net/ubuntu-cloud-archive/pike-staging/ubuntu xenial/main i386 Packages
        100 /var/lib/dpkg/status

Expected - openstack-dashboard installs with its stated dependencies satisfied
Instead - openstack-dashboard's post-install script failed due to an unmet dependency on apache2