Sorted pip requirements file to improve readability.

Moved pip requirements for tests into tests subdirectory and dropped
'pip-' prefix to shorten filenames and conform with common naming
scheme for pip requirements files.

Added constrains on OpenStack SDK 1.*.* to job ansible-collections-\
openstack-functional-devstack-releases on master branch because only
stable/1.0.0 branch is compatible to the OpenStack SDK 0.*.* series.

Changed job ansible-collections-openstack-functional-devstack-releases
on master branch to non-voting because OpenStack SDK 1.*.* has not
been released to PyPI yet, so tests on master branch are expected to
fail once we introduce breaking changes from stable/1.0.0 branch.

Change-Id: I6b6bb8c6900f7c8341bbf3f9a24999fbf693ba4b