keepalived: add track_script to vrrp_sync_group

keepalived: add track_script to vrrp_sync_group

Add logic to parse track_script section for vrrp_sync_group.
Keepalived supports script tracking in vrrp_sync_group but this was not
implemented by the uci implementation.

Note that if a vrrp_script is added to a sync group, you cannot use
priority/weight for that script as a vrrp_sync_group does not have a
priority/weight attached to it. It will do up/down as whole.

This option is optional and wouldn't affect any older configurations
during upgrade.

Signed-off-by: Rishabh <redacted>