Implement run_credentials_collector() in Puppet

Implement the `run_credentials_collector()` function of the concrete Puppet.

## Tasks
- [x] Modify the `IPuppet` interface to support running credentials collectors (0d) @mssalvatore 
    - [x] `IPuppet`
    - [x] `MockPuppet`
    - [x] `Puppet`
- [x] Modify the master to run credentials collectors (0d) @mssalvatore 
    - [x] `MockMaster`
    - [x] `AutomatedMaster`