dhcpv6-pd: T5387: add support for no-release flag

When no-release is specified, dhcp6c client will not release allocated address or prefix on client exit.

vyos.ifconfig: dhcpv6: T5387: re-use options_file for no release flag [WIP]

• Todo: render Jinja2 template and fill it

vyos.ifconfig: dhcpv6: T5387: finish options_file and no release flag in cli

vyos.ifconfig: dhcpv6: T5387: fix missing/wrong end tag

vyos.ifconfig: dhcpv6: T5387: fix options, no var for -n

dhcpv6-client: T5387: fix missing / from filepaths