Add spec for BSN L3 service plugin

Change-Id: I9fa36275b985ca3812d927343d7218de78b0ff36

specs/juno/bsn-l3-service-plugin.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+======================================================
+Big Switch - Convert L3 functions to L3 Service plugin
+======================================================
+
+
+https://blueprints.launchpad.net/neutron/+spec/bsn-l3-service-plugin
+
+Move the L3 functions from the Big Switch monolithic plugin to a separate L3
+service plugin so the fabric can provide L3 functionality for ML2 or the Big
+Switch Plugin from one common code base.
+
+Problem description
+===================
+There is no way to use the Big Switch L3 features when using the Big Switch
+ML2 driver.
+
+
+Proposed change
+===============
+
+Remove the L3 functions from the Big Switch core plugin and put them into an
+L3 service plugin. This will allow the Big Switch backend to provide L3 service
+when using the Big Switch plugin or the ML2 driver.
+
+
+Alternatives
+------------
+
+N/A
+
+Data model impact
+-----------------
+
+Database migration for L3 tables will be needed for new service plugin.
+
+
+REST API impact
+---------------
+N/A
+
+Security impact
+---------------
+N/A
+
+Notifications impact
+--------------------
+N/A
+
+Other end user impact
+---------------------
+N/A
+
+Performance Impact
+------------------
+N/A
+
+Other deployer impact
+---------------------
+An L3 service plugin configuration will be required for the BSN core plugin.
+
+Developer impact
+----------------
+
+N/A
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  kevinbenton
+
+Work Items
+----------
+
+* Separate code into L3 service plugin module
+* Re-organize BSN L3 tests into separate code from current unit test files
+
+Dependencies
+============
+
+This will be completed after the L3 offload functionality is done since that
+will require more L3 factoring and is a higher priority.[1]
+
+
+Testing
+=======
+
+The current unit tests and 3rd party CI will cover this change since no
+new features are being added.
+
+
+Documentation Impact
+====================
+The service plugin will have to be referenced in the configuration guide for
+the Big Switch plugin and the Big Switch ML2 agent.
+
+
+References
+==========
+
+1. https://blueprints.launchpad.net/neutron/+spec/bsn-blended-l3-agent