Monit HTTP calls should only retry 503 errors for 5min

- All other errors only retry for 20s
- Added a RetryClient with RetryStrategy to allow for more complex retry
  behavior to be independently tested
- Added a few testing utilities for fixtures and http body reading

[#77478708]

Signed-off-by: Karl Isenberg <kisenberg@pivotal.io>
fixed formatting

Signed-off-by: dmitriy kalinin <dmitriy@pivotal.io>
Moved golint and compile into bin/unit-test

Signed-off-by: dmitriy kalinin <dmitriy@pivotal.io>