vztmpl-dl: do not check GPG signatures by default

1. Set the built-in default for CHECK_TEMPLATE_SIG to no
2. Add --gpg-check option (and --update while we're at it)
3. Fix download.conf accordingly (and note the defaults)
4. Fix vztmpl-dl(8) man page accordingly

https://bugzilla.openvz.org/show_bug.cgi?id=2673

Reported-by: Pavel Odintsov <pavel.odintsov@gmail.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
3 files changed