Create constants for docker feed URLs

Create two new constants:

* DOCKER_V1_FEED_URL
* DOCKER_V2_FEED_URL

Use these new constants as appropriate. Test suite results both before
and after this change:

    =========  ==========  ==================
    Pulp Ver.  Num. Tests  Test Suite Results
    =========  ==========  ==================
    2.7        6           OK (skipped=3)
    dev (2.8)  12          OK (skipped=1)
    =========  ==========  ==================

Related to #123, "Reverse dockerhub feeds v1 and v2"

pulp_smash/constants.py

@@ -17,6 +17,18 @@
    http://pulp.readthedocs.org/en/latest/dev-guide/integration/rest-api/content/upload.html#creating-an-upload-request
 """
 
+DOCKER_V1_FEED_URL = 'https://index.docker.io'
+"""The URL to a V1 Docker registry.
+
+This URL can be used as the "feed" property of a Pulp Docker registry.
+"""
+
+DOCKER_V2_FEED_URL = 'https://registry-1.docker.io'
+"""The URL to a V2 Docker registry.
+
+This URL can be used as the "feed" property of a Pulp Docker registry.
+"""
+
 ERROR_KEYS = frozenset((
     '_href',
     'error',

pulp_smash/tests/docker/cli/test_sync.py

@@ -6,6 +6,7 @@
 from packaging.version import Version
 
 from pulp_smash import cli, config, selectors, utils
+from pulp_smash.constants import DOCKER_V1_FEED_URL, DOCKER_V2_FEED_URL
 
 _CREATE_COMMAND = (
     'pulp-admin docker repo create '
@@ -64,7 +65,7 @@ def setUpClass(cls):
         """Create and sync a docker repository with a v1 registry."""
         super(SyncV1TestCase, cls).setUpClass()
         kwargs = {
-            'feed': 'https://index.docker.io',  # v1 feed
+            'feed': DOCKER_V1_FEED_URL,
             'repo_id': cls.repo_id,
             'upstream_name': _UPSTREAM_NAME,
         }
@@ -86,7 +87,7 @@ def setUpClass(cls):
         if cls.cfg.version < Version('2.8'):
             raise unittest2.SkipTest('These tests require Pulp 2.8 or above.')
         kwargs = {
-            'feed': 'https://registry-1.docker.io',  # v2 feed
+            'feed': DOCKER_V2_FEED_URL,
             'repo_id': cls.repo_id,
             'upstream_name': _UPSTREAM_NAME,
         }
@@ -108,7 +109,7 @@ def setUpClass(cls):
         if cls.cfg.version < Version('2.8'):
             raise unittest2.SkipTest('These tests require Pulp 2.8 or above.')
         kwargs = {
-            'feed': 'https://registry-1.docker.io',  # v2 feed
+            'feed': DOCKER_V2_FEED_URL,
             'repo_id': cls.repo_id,
             'upstream_name': 'busybox',
         }