Change pipeline_manager to instance attribute in hooks

We should not assume the type of object which pecan.make_app() return
because it varies based on different configuration settings during the
test.

In order to achieve this, we have to change the PipelineHook's class
attribute pipeline_manager to be instance attribute, otherwise,
different api server created for different DB back-end would end up
sharing the same pipeline_manager which would break test tests.

(Failing unit tests in gate, manually booting it out. Please *DONT*
run recheck no bug on unit test failures).

Change-Id: Ic4370b1b8bec5723a0d719c74bdcdad8a6552912
Closes-Bug: #1327032
Merge "Change pipeline_manager to instance attribute in hooks"