Exceptions from the API were not properly caught when it uses
keystone session client path. As a result, error messages in the
response body from blazar REST APIs were not displayed.
This patch fixes it creating the new SessionClient class, which
is basically the same as its parent class, but it can raise
BlazarClientException if requests to the blazar server failed.
Change-Id: I98ba3949742f2a987beb7e16400608b8ebb60065
Closes-Bug: #1777548