Propagate incoming custom OSB-cmdb param to coab-vars.yml and filter it out from inner broker CSI

[gberche-orange]

Expected behavior

• As a coab service author,
• in order to propagate metadata from brokered service metadata (e.g. service-instance/org/space/user guid) to coab deployments (such as bosh vm meta-data)
• I need coab to propagate incoming OSB metadata provided as a x-osb-cmdb custom param (see https://github.com/orange-cloudfoundry/osb-cmdb-spike/issues/15) into coab-vars.yml
• And I need the x-osb-cmdb custom param to not be propagated to inner broker
• And I need the git commit message to include the x-osb-cmdb in content

Current behavior

• All params are propagated to inner brokers which may fail.

[gberche-orange]

Validated with the following commands that current inner brokers don't fail with additional extra unknown params passed:

cf cs p-mysql 10mb unbrokered  -c '{"x-osb-cmdb":4}' -b p-mysql
cf cs redis-unbrokered dedicated-vm -c '{"x-osb-cmdb":4}'  -b p-redis
cf cs p-rabbitmq37 standard redis-unbrokered -c '{"x-osb-cmdb":4}'  
cf cs mongodb default mongo-unbrokered -c '{"x-osb-cmdb":4}' -b p-mongodb