Add drenv.retry module

[nirs]

The module provides the on_error() utility, for retrying function calls up to number of retries with exponential backoff.

Example usage:

# Retry 5 times. The wait between the retries is doubled on every
# retry (1, 2, 4, 8, 16).
retry.on_error(func)

# Retry 10 times with up to 30 seconds between retries
# (1, 2, 4, 8, 16, 30, 30, 30, 30, 30).
retry.on_error(func, count=10)

# Retry 10 times, waiting 2 seconds between attempts.
retry.on_error(func, count=10, duration=2.0, factor=1.0)

# Retry only if raised error is an instance of foo.Error.
retry.on_error(func, count=3, error=foo.Error)