Alphabetically sort required packages, don't assume which is installed

Change-Id: I8b776dec1aeb343f455aef3a71516777c5c87685

vars/Debian.yml

@@ -15,12 +15,13 @@
 
 required_packages:
   - gcc
-  - python-dev
   - libffi-dev
   - libssl-dev
-  - python-setuptools
   - libxml2-dev
   - libxslt-dev
+  - python-dev
+  - python-setuptools
+  - which
 
 required_wsgi_packages:
   - apache2

vars/RedHat.yml

@@ -15,14 +15,15 @@
 
 required_packages:
   - gcc
+  - libffi-devel
+  - libselinux-python
+  - libxml2-devel
+  - libxslt-devel
+  - openssl-devel
   - python-devel
   - python-setuptools
-  - libffi-devel
-  - openssl-devel
   - redhat-rpm-config
-  - libxml2-devel 
-  - libxslt-devel
-  - libselinux-python
+  - which
 
 required_wsgi_packages:
   - httpd