No osci.yaml management

[nobuto-m]

osci.yaml has been updated when the charmhub migration happened some time ago.
https://github.com/openstack-charmers/release-tools/search?q=charm-unit-jobs-py38

However, since then release-tools hasn't have a way to keep it up-to-date. For example, the following update deleted the definition of testenv:py38 but oscsi.yaml in each charm still runs charm-unit-jobs-py38.

https://github.com/openstack-charmers/release-tools/pull/234/files#diff-787e12a93f77560a0ba72920b7c164e836e6d279b40e95cae43d2e2969b04fdd

If tox.ini is supposed to be managed centrally, osci.yaml should be managed in the same manner.