Don't delete PVC if it may be shared by others

[manics]

Closes #880

I've expanded test_delete_pvc to test changes in the pvc template.

[minrk]

Sorry for leaving this hanging for so long, but these aren't the only username-sensitive template fields (anymore). We could remove the leading { on both and it would capture {escaped_username}, {safe_user_server}, etc.. There is also the userid field.

Another, looser check would be to look for the strings username and user_server without the bran.

Another, more rigorous check is to use Formatter.parse to extract the fields and check them directly, rather than substring matching:

from string import Formatter

format_fields = [f[1] for f in Formatter().parse(s) if f[1] is not None]
user_sensitive = False
for (_literal_text, field_name, _format_spec, _conversion) in Formatter().parse(template):
    if field_name and 'user' in field_name:
        # template is user-sensitive
        user_sensitive = True

Otherwise, 👍 to the change.