bgpd: allow routes to be imported if the ES/ES-VRF is not present

bgpd: allow routes to be imported if the ES/ES-VRF is not present

In a sym-IRB setup the remote ES may not be installed if the tenant
VRF is not present locally. To allow that case while retaining the
fast-failover benefits for the case where the tenant VRF is locally
present we use the following approach -
1. If ES is present in the tenant VRF we use the L3NHG for installing
the MAC-IP based tenant route. This allows for efficient failover via
L3NHG updates.
2. If the ES is not present locally in the corresponding tenant VRF we
fall back to a non-NHG multi-path based routing approach. In this
case individual routes are updated when the ES links flap.

PS: #1 can be turned off entirely by disabling use-l3-nhg in BGP.

Ticket: CM-30935

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>