Daniel/add secrets manager

[Daniel-Han-Yang]

1. Added SM Module with unit test & docu, only supports ssh keys at the moment not HTTPS Personal Access token. Has additional config params that can be passed into the step file
2. Updated docker image to incorporate ssh-agent and ps command for testing
3. Updated actual sample git key for testing
4. Renamed environment.bats to run.bats which contains test for both environment and pre-exit hook, I've placed tests for both hooks together as the BATs test will not terminate if the ssh-agent continues to run Stuck on the last test of each file sstephenson/bats#80
   Hence, used pre-exit hook to ensure ssh-agent termination