[wheel] Don't spam logs with exceptions when checks aren't wheels

[olivielpeau]

What does this PR do?

Avoids spamming logs with exceptions when checks aren't wheels.

We always try to load the checks as wheel-installed checks, but it's
not an error if the checks are installed differently, so avoid logging with
exception level.

Motivation

Without this, we get this error in the logs for every check that's enabled, on agent startup:

2017-12-29 13:12:34 UTC | ERROR | dd.collector | config(config.py:872) | Unable to import check module process from site-packages
Traceback (most recent call last):
  File "/opt/datadog-agent/agent/config.py", line 869, in _get_check_module
    check_module = import_module("datadog_checks.{}".format(check_name))
  File "/opt/datadog-agent/embedded/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
ImportError: No module named datadog_checks.process

[truthbk]

Awesome! Thanks! 👌