Allow overriding grafana_package from vars

[kunickiaj]

Conditionally import default os-specific package names if grafana_package isn't explicitly set.

Updates the molecule alternative tests to use the grafana-enterprise repo to allow for also verifying that overriding grafana_package to grafana-enterprise works.

Resolves #228

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This change is 

[kunickiaj]

This was the cleanest way I could come up with, but if there's a better way you can think of I'll welcome it.

[kunickiaj]

The linter issues failing CI are not files I've changed. Also noted in local testing that everything works except centos8 which seems to be unrelated:

fatal: [centos8]: FAILED! => {"attempts": 5, "changed": false, "msg": "Failed to validate GPG signature for libSM-1.2.3-1.el8.x86_64"}

[paulfantom]

Could you rebase it so CI can test this PR?

[kunickiaj]

This still looks like the centos8 failure with

Failed to validate GPG signature for libSM-1.2.3-1.el8.x86_64

Not sure what that's about but I don't believe its related to this change.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[kunickiaj]

Will try to rebase this again if the centos8 issue has been resolved.

[kunickiaj]

It looks like the centos8 issue is still present.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[SuperQ]

This role has been deprecated in favor of a the prometheus-community/ansible collection.