docs: Fix plugin registry generation failures

This patch set fixes plugin registry generation script which
is resulting in the documentation page ending up empty [0].

This is because the script `tools/generate-tempest-plugins-list.sh`
is using an old flag (`-A`).

This drops the `declare -A plugins` command because `plugins` isn't
used anywhere and also uses `decode('utf-8')` on the initial response
lookup for all the projects which is done already in the script
elsewhere [2].

[0] https://docs.openstack.org/tempest/latest/plugin-registry.html
[1] https://bash.cyberciti.biz/bash-reference-manual/Bash-Builtins.html#index-declare-132
[2] https://github.com/openstack/tempest/blob/da8aec9aedcac940ba1dbc3beee50f05f13af3f1/tools/generate-tempest-plugins-list.py#L67

Change-Id: I70b59c869df06f35c0f7ca6d20e3584502cda8ef
2 files changed