Support yum priorities

[skpy]

With Red Hat now shipping Gluster in the official RHEL channels, there can be version problems when using the upstream Gluster repos.

One suggested fix is to use yum priorities.

We can easily add a priority element to the yumrepo resource, but this raises some interesting questions:

Should we install the yum-plugins-priorities package if a priority is defined?
I'm leaning toward 'yes'.

Should we expose a new repo_version parameter?
I have working code not yet committed that will parse the full package version (X.Y.X-1.el6) to create the appropriate yumrepo resource, but such a version hard-codes the RHEL major release, which makes a single Hiera definition for Gluster version hard for environments running both RHEL6 and RHEL7.

A version of '3.5.2' will work for the repo creation, but will fail to actually install any packages:

Error: Could not update: Failed to update to version 3.5.2, got version 3.5.2-1.el6 instead
Error: /Stage[main]/Gluster::Install/Package[glusterfs-fuse]/ensure: change from absent to 3.5.2 failed: Could not update: Failed to update to version 3.5.2, got version 3.5.2-1.el6 instead

We could keep the current implementation whereby the declaration of the repo controls the version that will be applied. This does seem to work as expected with priorities: if the Gluster repo has priority=50, then a simple yum install glusterfs-server will find the version specified in the Gluster repo, not the RHEL repo.

[skpy]

Specifying the repo version but not specifying a package version does not work, either.

It appears that the best viable solution is to specify the full version to install and parse the repo details from that.

Which means that users will need to handle el6 and el7 on their own in some way.

[skpy]

A potential solution for supporting multiple major RHEL releases without a lot of work in this module would be to use Hiera interpolation in the following way:

gluster::version: "3.5.2-1.el%{::operatingsystemmajrelease}"

[skpy]

Resolved with PR #5