add source config for the charm

src/config.yaml

@@ -1,4 +1,9 @@
 options:
+  source:
+    default: ppa:canonical-arista/queens-stable
+    type: string
+    description: |
+      Repository from which to install. Ex: ppa:canonical-arista/stable
   arista-version:
     default: '2017.2.2'
     type: string

src/lib/charm/openstack/neutron_arista.py

@@ -11,8 +11,6 @@ from charmhelpers.core.hookenv import (
 )
 import charms_openstack.charm
 
-from charmhelpers.contrib.python.packages import pip_install
-
 from charmhelpers.contrib.openstack import (
     context,
     templating,