Refactor WSGI.middleware and imporve test coverage

*) Move all logic related to secure transport of trace info to osprofiler/utils.py
*) Add to methods singed_pack, signed_unpack that hides all complexity of packing
data, generating hmac, and validating hmac data.
*) Cover osprofiler/utils.py by separated tests
*) Remove mess from WSGI middleware and add_trace_id_header
*) Simplify web tests
*) Disable possibility to setup empty hmac keys, cause of security reasons

Change-Id: I86112eb0b6f0f01222a4f84985edd74a1881e89a