Service plan and params support

[gberche-orange]

As a service author, in order to support arbitrary params in my service, I need to receive content of arbitrary params in a format by bosh deployment can leverage.

Proposed solution: the coab-vars.yml file contains the open service broker api standardized input for both service provisionning and update.

---
deployment_name: "c_aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaa0"
instance_id: "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaa0"
service_id: "service_definition_id"
plan_id: "plan_guid"
context:
  platform: "cloudfoundry"
  user_guid: "user_guid1"
  space_guid: "space_guid1"
  organization_guid: "org_guid1"
parameters:
  cacheRatio: 0.8642
  cacheSizeMb: 10
  slowQuery: false
previous_values:
  plan_id: "previous_plan_guid"

Service authors can test in their models various content of the coab-vars.yml which will be safely ignored by COAB

To maintain security, Input validation is applied on the name and value of arbitrary params to prevent various injections (such as reading from credhub, spruce, file system injection, yml loading or reference expansions), and only alphabetical characters, numbers are supported

Out of scope: implement the GET endpoints.