factoring out regex for vztmpl

factoring out regex for vztmpl

stores the regex definition in PVE::Storage.

One test had to be adapted because it tested obsolete code. Namely:
it expects vztmpl to only end with .tar.gz, but the new regex also
includes .tar.xz, there is nothing against allowing .tar.xz files as
vztmpl files.

Signed-off-by: Lorenz Stechauner <l.stechauner@proxmox.com>