func: Don't use private method of AsyncProcess

func: Don't use private method of AsyncProcess

In functional test we simulate crash of AsyncProcess by calling
_kill_process(). This method is a private method and such usage
introduced a race where process was respawned prior to calling wait() of
killed process, leading to infinite wait on newly spawned process.

This patch adds manual send of kill and then active waiting for process
to be respawned, similarly like done with recent keepalived patch [1].

[1] https://review.openstack.org/#/c/222460/7/neutron/tests/functional/agent/linux/test_keepalived.py

Closes-Bug: #1477860
Change-Id: I1c91393304d65a0695311416ecc5b64fd549b192