Optionally create the vault user in packaging post-install

[stevendpclark]

• Only create the vault user if the useradd command is present on the system, similarly for the purge option on the package removal, call userdel only if the command is present on the system

• This will help rpm/deb installation on minimal containers that people want to use the vault as a command line application and not as a server.

DRAFT: I'm creating this PR to see the resulting rpm/deb's created by CRT