Provide IPAM feature

[gberche-orange]

Expected behavior

As service author

• in order to provision services that require IP addresses beyond bosh IPAM support (typically public floating IP address, see https://github.com/orange-cloudfoundry/paas-templates/issues/676)
• I need COAB to provide basic IPAM function (provide one or more IP addresses obtained from a pool during service provisionning, and release these IP addresses during service unprovisionning)

Observed behavior

COAB does not provide yet such feature

Possible design

• Use paas-secret git repo as the backing storage for IPAM
• provide a coab-ip-vars.yml in paas-templates service instance for use by the model
• Alternative IPAM storage format
  • One file per IP possibly with symlink to associated deployment (to help detecting leaks)
    • See Inspiration from https://github.com/concourse/pool-resource#git-repository-structure
  • One single file with one entry by IP
• Software design
  • SecretsGenerator and TemplateGenerator handle new provisionManagedIps() and releaseManagedIps()
  • Add retry mechanism for handling conflicts
    • SimpleGitManager throws RetryException upong conflict during git push
    • ProcessorChain to support retries when catching RetryException
      • Potentially cloning the original Context object to have independent processing chain executions