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/nova b/files/debs/nova
index e5110e9..dce8f6a 100644
--- a/files/debs/nova
+++ b/files/debs/nova
@@ -16,7 +16,7 @@
 mysql-server # NOPRIME
 parted
 pm-utils
-python-mysqldb
+python3-mysqldb
 qemu # dist:wheezy,jessie NOPRIME
 qemu-kvm # NOPRIME
 rabbitmq-server # NOPRIME