Move to dnf interface for RHCOS package install

Feature Overview (aka. Goal Summary)  

Now that dnf has a shim layer to manage packages on ostree systems, we should incorporate it as the correct way to write Containerfiles for RHCOS.

Goals (aka. expected user outcomes)

The goal is to give RHEL administrators a familiar interface for adding packages to their customized RHCOS builds.

Requirements (aka. Acceptance Criteria):

• A supported version of dnf is available in the on-cluster build environment
• Customers can get a supported version of dnf that can handle off-cluster builds
• Documentation and public examples should start using dnf
• A plan for deprecating rpm-ostree

Documentation Considerations

Provide information that needs to be considered and planned so that documentation will meet customer needs.  If the feature extends existing functionality, provide a link to its current documentation. Initial completion during Refinement status.

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