Amend python pkgs for debian-based distros

Drop python2 packages that are no longer needed. Replace other py2 pkgs
with their py3 variant.

Change-Id: Id5fc11d1ae0a7daf439c7d72f831f298f5d0ce31
diff --git a/files/debs/keystone b/files/debs/keystone
index fd0317b..1cfa6ff 100644
--- a/files/debs/keystone
+++ b/files/debs/keystone
@@ -2,5 +2,5 @@
 libldap2-dev
 libsasl2-dev
 memcached
-python-mysqldb
+python3-mysqldb
 sqlite3