[swss] ecmp hash seed configurable from config_db

Before this commit, the ECMP hash seed could only be changed through to
indirect parameters in DEVICE_METADATA:
- type (10 for LeafRouter, 25 for SpineRouter, etc...)
- namespace_id: the value is used as an offset

This commit brings the possibility to enforce the ECMP hash seed from
config_db.json via the new 'ecmp_hash_seed' field. If defined:
- the value deduced from the 'type' is ignored
- the namespace_id is still used as an offset

It is particularly useful to fix ECMP polarization issue, especially
when having more than 3 layers in a clos matrix.

dockers/docker-orchagent/switch.json.j2

@@ -2,7 +2,9 @@
 {# set default hash seed to 0 #}
 {% set hash_seed = 0 %}
 {% set hash_seed_offset = 0 %}
-{% if DEVICE_METADATA.localhost.type %}
+{% if DEVICE_METADATA.localhost.ecmp_hash_seed %}
+{% set hash_seed = DEVICE_METADATA.localhost.ecmp_hash_seed | int %}
+{% elif DEVICE_METADATA.localhost.type %}
 {% if "ToRRouter" in DEVICE_METADATA.localhost.type or DEVICE_METADATA.localhost.type in ["EPMS", "MgmtTsToR"] %}
 {% set hash_seed = 0 %}
 {% elif "LeafRouter" in DEVICE_METADATA.localhost.type %}