Expose X-Trace-* constants
Expose X-Trace-* constants

For users that want to know what the currently used
trace headers are in a progamatic way it is nice to
expose these headers as constants that others can
easily import.

This makes it possible to do things like:

    >>> from osprofiler import web
    >>> web.X_TRACE_INFO
    'X-Trace-Info'
    >>> web.X_TRACE_HMAC
    'X-Trace-HMAC'

Change-Id: Ibd9de5dde77b5a0f8b37b23fefc4ed6167f6a00a

openstack/osprofiler,stackforge/osprofiler,stackforge/osprofiler,openstack/osprofiler,stackforge/osprofiler,openstack/osprofiler