This commit makes to generate a plugin list for the registry
document and, also fixes a python3 incompatibility and docs warnings
in generate-grenade-plugins-list.sh.
And this commit also removes doc/source/plugin-registry.rst because
it's generated by the script.
Change-Id: I391ca452f2e99e899ded8e4bcb5649a4511a7696
7518301120