RPM Support

DNF fails to synchronize cache for synced repositories.

## On server1:
$ pulp-admin rpm repo create --repo-id=ius-stable-el7 --feed=https://dl.iuscommunity.org/pub/ius/stable/CentOS/7/x86_64/ --relative-url=ius-stable-el7
Successfully created repository [ius-stable-el7]

$ pulp-admin rpm repo sync run --repo-id=ius-stable-el7
<< snip >>

## At client:
$ cat /etc/yum.repos.d/ius.repo
[ius-stable-el7]
name=ius-stable-el7
baseurl=https://server2/pulp/repos/ius-stable-el7/
enabled=1
skip_if_unavailable=0
gpgcheck=0

# dnf -v update
DNF version: 2.7.5
cachedir: /var/cache/dnf
<< snip >>
Cannot download 'https://server2/pulp/repos/ius-stable-el7/': Yum repo downloading error: Downloading error(s): repodata/f9cabc6ee185c75cf81c6b5eb6fc62a7d32e41b02bce2c5d9ad42935ecd112df-modules.yaml.gz - Cannot download, all mirrors were already tried without success.
Error: Failed to synchronize cache for repo 'ius-stable-el7'

Same on fedora28/29 and centos 7 clients.

The apache log of server2 shows that *-models.yaml.gz was returned with HTTP 200.
models.yaml is empty, tho. The feed has no models file at all.

# Server1:
$ uname -a
Linux server1 3.10.0-862.14.4.el7.x86_64 #1 SMP Wed Sep 26 15:12:11 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
$ rpm -qa | grep pulp
python-pulp-puppet-common-2.18.0-1.el7.noarch
python-pulp-deb-common-1.8.0-1.el7.noarch
python-pulp-bindings-2.18.0-2.el7.noarch
python-pulp-python-common-2.0.3-1.el7.noarch
python-pulp-repoauth-2.18.0-2.el7.noarch
python-pulp-agent-lib-2.18.0-2.el7.noarch
pulp-python-plugins-2.0.3-1.el7.noarch
pulp-puppet-tools-2.18.0-1.el7.noarch
python-pulp-oid_validation-2.18.0-2.el7.noarch
python-pulp-rpm-common-2.18.0-1.el7.noarch
python-pulp-docker-common-3.2.2-1.el7.noarch
pulp-deb-plugins-1.8.0-1.el7.noarch
python-pulp-client-lib-2.18.0-2.el7.noarch
pulp-docker-admin-extensions-3.2.2-1.el7.noarch
python-pulp-ostree-common-1.3.1-1.el7.noarch
pulp-python-admin-extensions-2.0.3-1.el7.noarch
pulp-server-2.18.0-2.el7.noarch
python-pulp-common-2.18.0-2.el7.noarch
pulp-rpm-admin-extensions-2.18.0-1.el7.noarch
pulp-ostree-plugins-1.3.1-1.el7.noarch
python-pulp-streamer-2.18.0-2.el7.noarch
pulp-selinux-2.18.0-2.el7.noarch
pulp-admin-client-2.18.0-2.el7.noarch
pulp-rpm-plugins-2.18.0-1.el7.noarch