keepalived: use section vrrp_script for option track_script

keepalived: use section vrrp_script for option track_script

`track_script` and `vrrp_script` are both sections that run custom scripts
which handle priority of a vrrp_instance.
`track_script` is not supported by this uci implementation

`vrrp_instance` was still trying to fetch config for track_script from section
'track_script'.

After the changes, when listing track_script in vrrp_instance,
it tries to fetch config from section `vrrp_script` which is supported.

Signed-off-by: Rishabh <redacted>