Adding environment variable record injection

Per the discussion on https://github.com/github/octodns/issues/583
here is a work in progress of environment variable injection for
discussion.

tests/test_octodns_source_envvar.py

@@ -0,0 +1,34 @@
+from six import text_type
+from unittest import TestCase
+from unittest.mock import patch
+
+from octodns.source.envvar import EnvVarSource
+from octodns.source.envvar import EnvironmentVariableNotFoundException
+from octodns.zone import Zone
+
+
+class TestEnvVarSource(TestCase):
+
+    def test_read_variable(self):
+        envvar = 'OCTODNS_TEST_ENVIRONMENT_VARIABLE'
+        source = EnvVarSource('testid', envvar, 'recordname', ttl=120)
+        with self.assertRaises(EnvironmentVariableNotFoundException) as ctx:
+            source._read_variable()
+        msg = 'Unknown environment variable {}'.format(envvar)
+        self.assertEquals(msg, text_type(ctx.exception))
+
+        with patch.dict('os.environ', {envvar: 'testvalue'}):
+            source._read_variable()
+        self.assertEquals(source.value, 'testvalue')
+
+    def test_populate(self):
+        envvar = 'TEST_VAR'
+        value = 'somevalue'
+        record = 'testrecord'
+        source = EnvVarSource('testid', envvar, record)
+        zone = Zone('unit.tests.', [])
+
+        with patch.dict('os.environ', {envvar: value}):
+            source.populate(zone)
+
+        # TODO: Validate zone and record