Federation functional job failing on Bionic

When doing a test to migrate the functional tests to Bionic, this error occured within keystone-dsvm-functional-federation:

2018-10-18 15:43:32.336409 | controller | ++ functions-common:apt_get:1083 : sudo DEBIAN_FRONTEND=noninteractive http_proxy= https_proxy= no_proxy= apt-get --option Dpkg::Options::=--force-confold --assume-yes install libapache2-mod-shib2
2018-10-18 15:43:32.376149 | controller | Reading package lists...
2018-10-18 15:43:32.579166 | controller | Building dependency tree...
2018-10-18 15:43:32.579865 | controller | Reading state information...
2018-10-18 15:43:32.664285 | controller | Some packages could not be installed. This may mean that you have
2018-10-18 15:43:32.664482 | controller | requested an impossible situation or if you are using the unstable
2018-10-18 15:43:32.664634 | controller | distribution that some required packages have not yet been created
2018-10-18 15:43:32.664719 | controller | or been moved out of Incoming.
2018-10-18 15:43:32.664856 | controller | The following information may help to resolve the situation:
2018-10-18 15:43:32.664886 | controller |
2018-10-18 15:43:32.665000 | controller | The following packages have unmet dependencies:
2018-10-18 15:43:32.738989 | controller | libapache2-mod-shib2 : Depends: libshibsp-plugins (= 2.6.1+dfsg1-2) but it is not going to be installed
2018-10-18 15:43:32.739266 | controller | Depends: shibboleth-sp2-utils (>= 2.6) but it is not going to be installed
2018-10-18 15:43:32.739445 | controller | Depends: libshibsp7 but it is not going to be installed
2018-10-18 15:43:32.739651 | controller | Depends: libxmltooling7 (>= 1.6.0-5) but it is not going to be installed
2018-10-18 15:43:32.756003 | controller | E: Unable to correct problems, you have held broken packages.

Tracing the sequence of dependencies, it seems that ... isn't installable on ubuntu-server Bionic because of this reference to an outdated library:

$ sudo apt install libxmltooling7
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libxmltooling7 : Depends: libcurl3 (>= 7.16.2) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
$ sudo apt install libcurl3
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
  grub-pc-bin
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  curl libcurl4 pollinate ubuntu-server
The following NEW packages will be installed:
  libcurl3
0 upgraded, 1 newly installed, 4 to remove and 15 not upgraded.
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