Debian 9+ & Ubuntu 18+ do not appear to install an `gnupg` or equivalent package by default, therefore we need to install it instead, as it is required for importing the GPG keys.
This is the only package required to down they keys and also, there's no need to include `gnupg` package explicitly as its required by the distribution anyway.
Installing just `gnupg-curl` causes this issue:
```
Setting up ca-certificates (20170717~16.04.2) ...
Setting up krb5-locales (1.13.2+dfsg-5ubuntu2.1) ...
Setting up libsasl2-modules:amd64 (2.1.26.dfsg1-14ubuntu0.1) ...
Setting up gnupg-curl (1.4.20-1ubuntu3.3) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Processing triggers for ca-certificates (20170717~16.04.2) ...
Updating certificates in /etc/ssl/certs...
148 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d...
done.
* ERROR: Failed to run install_ubuntu_stable_deps()!!!
* DEBUG: Removing the logging pipe /tmp/bootstrap-salt.logpipe
* DEBUG: Removing the temporary apt error file /tmp/apt_error.n3jI
ERROR: Service 'ubuntu-16-tester' failed to build: The command '/bin/sh -c /tmp/bootstrap-salt.sh -X -d -D' returned a non-zero code: 1
```
Where as installing both `gnupg` and `curl` ensures `install_ubuntu_stable_deps` runs correctly, as a sub-dependency of `curl` looks to be missing.
```
Setting up ca-certificates (20170717~16.04.2) ...
Setting up krb5-locales (1.13.2+dfsg-5ubuntu2.1) ...
Setting up libsasl2-modules:amd64 (2.1.26.dfsg1-14ubuntu0.1) ...
Setting up curl (7.47.0-1ubuntu2.13) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Processing triggers for ca-certificates (20170717~16.04.2) ...
Updating certificates in /etc/ssl/certs...
148 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d...
done.
OK
```
I suspect this is a typo, because looking at the code line above, `__PACKAGES="${__PACKAGES} gnupg dirmngr"`, that installs two packages.
