Install Enterprise via repo

[lakrass]

• Add ability to install Vault Enterprise via HashiCorp repo
• Set the default value of the vault_version_repo_suffix variable according to the vault_enterprise variable
• Notify the service restart handler on installation via HashiCorp repo
• Removed check of non-existent variable "vault_install_remote_repo" in tasks/main.yml
• Preparations to release v2.5.9

[sschmittsva]

Hi!

Thanks for your PR. I would like to split out the Nix parts to have a minimal PR.

[lakrass]

Hey @sschmittsva,

I included your latest changes from the master in this PR, responded to your comments and fixed a few things I agree with.
Feel free to check my PR again.

We should discuss on releasing v2.5.9 with this PR. If we do a release by this PR, we should remove the "- WIP" from the changelog. If you plan other changes before releasing the next version, we could keep the "- WIP".

Thank you for your help! :)

[lakrass]

I added two more small changes:

• set the default value of the vault_version_repo_suffix variable according to the vault_enterprise variable
• notify the service restart handler on installation via HashiCorp repo

[FalcoSuessgott]

2 minor things before merging :)

[lakrass]

Hello @FalcoSuessgott, thank you for your review. :)
I just fixed the remarks you issued.

[FalcoSuessgott]

LGTM Atze :)