Replace openssl-version configuration with variant

[scalvert]

This PR adds a new variant input, which allows consumers to further qualify the version of Volta to target, specifically in the case of RHEL installations.

[rwjblue]

I'd prefer that variant be everything after volta-${version}. So a valid value would be linux-openssl-rhel for example.

We should also update openssl-version to allow specifying rhel/RHEL specifically (so you could just say that too).

[scalvert]

I was also thinking along similar lines. It felt odd to have both openssl-version and variant when they're both so closely related. It may make sense instead to just consolidate them, and allow users to do what you described above. WDYT?

[rwjblue]

I split out the actions-docs part into #104