Rename package lists (and related vars) appropriately

In order to make it easier to differentiate between the lists of
python packages, distribution packages, downloaded packages,
package pins and other similar variables the variable names are
being changed to ensure that they have a more explicit suffix
that defines the purpose and makes the naming more consistent.

This is to facilitate a lookup plugin which will be able to look
up all the package lists and present them as a consolidated piece
of data which may be used for artifact preparation.

Change-Id: I2c0d397df559df1725ec541afd7616e114d49d6f

releasenotes/notes/package-list-name-changes-e6f88d12f3bd9fa0.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - The variable ``swift_apt_packages`` has been renamed to
+    ``swift_distro_packages``.

tasks/install-apt.yml

@@ -34,4 +34,4 @@
   until: install_packages|success
   retries: 5
   delay: 2
-  with_items: "{{ swift_apt_packages }}"
+  with_items: "{{ swift_distro_packages }}"

vars/debian.yml

@@ -15,7 +15,7 @@
 
 cache_timeout: 600
 
-swift_apt_packages:
+swift_distro_packages:
   - curl
   - gcc
   - git-core