Remove python cryptography libraries in favor of subprocess to `openssl`

[addyess]

Trying to deploy this charm to arm64 resulted in something fairly broken

Much of it was the fact that charmcraft packs binary packages into the charm for cryptography. That library was driven by the charm's need to generate a self-signed cert.

So -- this PR addresses the inclusion of cryptography by using openssl and subprocess to generate certs in an arch agnostic way.

[jnsgruk]

I might be tempted to remove this functionality completely and just implement the tls-certificates interface - that way the charm will be able to get certs from vault, or statically from the tls-certificates-operator

[addyess]

@jnsgruk yes, i will do that in a following PR. I have the code prepared for that, just wanted to wait for a separate PR