Require support for multiple (track-interface, priority-decement) configuration under VRRP

[abinmathewabraham]

Currently the track-interface (https://github.com/openconfig/public/blob/master/release/models/interfaces/openconfig-if-ip.yang#L683) is a leaf-list. But the priority-decrement config is modeled as a leaf (https://github.com/openconfig/public/blob/master/release/models/interfaces/openconfig-if-ip.yang#L700). The requirement is to have multiple (track-interface, priority-decrement) configuration. Example:

track-interface Gig1 priority-decrement 10
track-interface Gig2 priority-decrement 20
track-interface Gig3 priority-decrement 30

In my opinion, the holder holding track-interface and priority-decrement should be list with track-interface as the key.