fix(README): Document prerequisites and common variables for deployment

[m-czernek]

What does this PR change?

In this PR, I am documenting:

• Libvirt prerequisites: this seemed reasonable to me since the section also contains description of installing the terraform-provider-libvirt package
• Variables: a newcomer to Uyuni probably won't know what CC stands for. Additionally, the images section might be helpful

[meaksh]

Thanks a lot for these improvements! 🎉

I just made some suggestions that maybe you could consider. Thanks in advance!

[agraul]

There is also #769 which covers some of the same changes. Maybe we should also take a look at that one.

[m-czernek]

@agraul I think this PR supersedes changes in #769 , doesn't it? IMO the cc_* variables are necessary for any backend, so it probably doesn't make sense to document them in the libvirt module. Perhaps we can delete the current cc docs in https://github.com/uyuni-project/sumaform/blob/master/backend_modules/libvirt/README.md?

complete the cc_password variable in the base module

[meaksh]

Looks good, thanks!