Make an auto-generated support matrix

Trove supports quite a lot database backends, and each of them has its capabilities. But there is still lack of a distinct document to show the capability gaps between different datastores. And a "static" document may be out of sync with code.
I am inspired by nova's support matrix[1]. This is generated by support-matrix.ini[2] via support_matrix.py[3].
When someone pushed an feature-related patch to gerrit, core review should check if it changed the support matrix(like [4]).

This would helps us focusing on the missing feature.

Related: https://bugs.launchpad.net/trove/+bug/1543555

[1] https://docs.openstack.org/nova/latest/user/support-matrix.html
[2] https://github.com/openstack/nova/blob/master/doc/source/user/support-matrix.ini
[3] https://github.com/openstack/nova/blob/master/doc/ext/support_matrix.py
[4] https://review.openstack.org/#/c/450636/