Add concept of host alias for regrouping hosts

Ansible doesn't provide a way to uniquely identify hosts which would
allow ARA to track a single host across multiple playbook runs.

This patch adds a field for each host, 'alias'.
The plan is to leverage this field as a way to regroup the results
for what is believed to be the same host -- the same alias.

A host alias is arbitrary and generic. It can be the inventory
hostname or the result of a hashing algorithm like md5sum/sha1sum
of a combination of host facts.

Change-Id: I16b668f473a9f31686c7a921d7dcd31a5292febd

ara/api/migrations/0003_host_alias.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.1.1 on 2018-09-04 00:14
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('api', '0002_add_labels'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='host',
+            name='alias',
+            field=models.CharField(max_length=255, null=True),
+        ),
+    ]

ara/api/models.py

@@ -194,6 +194,16 @@ class Host(Base):
     ok = models.IntegerField(default=0)
     skipped = models.IntegerField(default=0)
     unreachable = models.IntegerField(default=0)
+    # Ansible doesn't supply a mechanism to uniquely identify a host out of
+    # the box.
+    # ARA can attempt to reconcile what it believes are the results same hosts
+    # based on user-supplied configuration or through a hashing algorithm.
+    # The goal is to "regroup" all unique hosts under a single alias if they
+    # are the same host.
+    # The logic for supplying aliases does not live here, it's provided by the
+    # clients and consumers.
+    alias = models.CharField(max_length=255, null=True)
+
     play = models.ForeignKey(Play, on_delete=models.DO_NOTHING, related_name='hosts')
 
     def __str__(self):

ara/api/tests/factories.py

@@ -109,6 +109,7 @@ class HostFactory(factory.DjangoModelFactory):
     ok = 2
     skipped = 1
     unreachable = 0
+    alias = "9f5d3ba7-e43d-4f3b-ab17-f90c39e43d07"
     play = factory.SubFactory(PlayFactory)