Add option --date-iso-format for snapshot names

[apprell]

Hi, thanks for your interest in this script.

It's fine, but I don't really like that the name contradicts the allowed characters in Proxmox when you create a snapshot from the web interface

I'd replace all the "-" with "_".

suffix = "_" + suffix_datetime.isoformat(timespec="seconds").replace("-", "_").replace(":", "_")

snapshot = re.search(r'auto{0}(\d+|_\d{{4}}_\d{{2}}_\d{{2}}T\d{{2}}_\d{{2}}_\d{{2}})'.format(label),
                     snapshot.replace('`->', '').split()[0])

And then the name would fit the requirement of Proxmox I understand that it already works, but I would stick to the requirements of Proxmox

[jocelynj]

You're right - I couldn't find an explicit list of allowed characters, but should have thought of the web interface. I will push a change.