[bgp]: Enable next-hop-tracking through default (#5600)

**- Why I did it**
FRR introduced [next hop tracking](http://docs.frrouting.org/projects/dev-guide/en/latest/next-hop-tracking.html) functionality.
That functionality requires resolving BGP neighbors before setting BGP connection (or explicit ebgp-multihop command). Sometimes (BGP MONITORS) our neighbors are not directly connected and sessions are IBGP. In this case current configuration prevents FRR to establish BGP connections.  Reason would be "waiting for NHT". To fix that we need either add static routes for each not-directly connected ibgp neighbor, or enable command `ip nht resolve-via-default`

**- How I did it**
Put `ip nht resolve-via-default` into the config

**- How to verify it**
Build an image. Enable BGP_MONITOR entry and check that entry is Established or Connecting in FRR

Co-authored-by: Pavel Shirshov <pavel.contrib@gmail.com>

Signed-off-by: Abhishek Dosi <abdosi@microsoft.com>

dockers/docker-fpm-frr/frr/zebra/zebra.interfaces.conf.j2

@@ -10,6 +10,8 @@ vni {{ vnet_metadata['vni'] }}
 {% endblock vrf %}
 !
 {% block interfaces %}
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 {% for (name, prefix) in INTERFACE|pfx_filter %}
 interface {{ name }}

src/sonic-bgpcfgd/tests/data/sonic-cfggen/frr.conf.j2/all.conf

@@ -15,6 +15,8 @@ log facility local4
 ! end of template: common/daemons.common.conf.j2!
 agentx
 !
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface Ethernet0
 link-detect

src/sonic-bgpcfgd/tests/data/sonic-cfggen/zebra/interfaces.conf

@@ -1,4 +1,7 @@
 !
+!
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface Ethernet0
 link-detect

src/sonic-bgpcfgd/tests/data/sonic-cfggen/zebra/zebra.conf

@@ -20,6 +20,8 @@ vni 10
 vrf Second
 vni 20
 !
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface Ethernet0
 link-detect

src/sonic-config-engine/tests/sample_output/frr.conf

@@ -18,6 +18,8 @@ agentx
 !
 !
 !
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface PortChannel01
 link-detect

src/sonic-config-engine/tests/sample_output/t2-chassis-fe-pc-zebra.conf

@@ -12,6 +12,8 @@ vrf VnetFE
 vni 8000
 !
 !
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface PortChannel0
 link-detect

src/sonic-config-engine/tests/sample_output/t2-chassis-fe-vni-zebra.conf

@@ -19,6 +19,8 @@ vrf VnetFE
 vni 9000
 !
 !
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface Ethernet0
 link-detect

src/sonic-config-engine/tests/sample_output/t2-chassis-fe-zebra.conf

@@ -19,6 +19,8 @@ vrf VnetFE
 vni 8000
 !
 !
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface Ethernet0
 link-detect

src/sonic-config-engine/tests/sample_output/zebra_frr.conf

@@ -16,6 +16,8 @@ log facility local4
 ! end of template: common/daemons.common.conf.j2!
 !
 !
+! Enable nht through default route
+ip nht resolve-via-default
 ! Enable link-detect (default disabled)
 interface PortChannel01
 link-detect