Implement testing for billing

[meherhendi]

write unit tests that ensure billing is done correctly in these situations:

• under plan model:

  • project reaches billing date with sufficient balance to be deducted from
  • project reaches billing date with insufficient credit to be deducted from and should not be renewed to another billing cycle

• under pay-per-use model:

  • project reaches critical balance => user should be notified , provisioner should hibernate cloud resources

• under both models:

  • user stops his plan and resumes after x number of days