Extend Flask Blueprint class to allow skipping routes

When adding a new API we want to use the old code for any routes that
aren't being overridden.

This modifies the Flask Blueprint class so that a skip_rules list can be
passed to server.register_blueprint()

src/pylorax/api/flask_blueprint.py

@@ -0,0 +1,54 @@
+#
+# Copyright (C) 2019 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+""" Flask Blueprints that support skipping routes
+
+When using Blueprints for API versioning you will usually want to fall back
+to the previous version's rules for routes that have no new behavior. To do
+this we add a 'skip_rule' list to the Blueprint's options dictionary. It lists
+all of the routes that you do not want to register.
+
+For example:
+    from pylorax.api.v0 import v0
+    from pylorax.api.v1 import v1
+
+    server.register_blueprint(v0, url_prefix="/api/v0/")
+    server.register_blueprint(v0, url_prefix="/api/v1/", skip_rules=["/blueprints/list"]
+    server.register_blueprint(v1, url_prefix="/api/v1/")
+
+This will register all of v0's routes under `/api/v0`, and all but `/blueprints/list` under /api/v1,
+and then register v1's version of `/blueprints/list` under `/api/v1`
+
+"""
+from flask import Blueprint
+from flask.blueprints import BlueprintSetupState
+
+class BlueprintSetupStateSkip(BlueprintSetupState):
+    def __init__(self, blueprint, app, options, first_registration, skip_rules):
+        self._skip_rules = skip_rules
+        super(BlueprintSetupStateSkip, self).__init__(blueprint, app, options, first_registration)
+
+    def add_url_rule(self, rule, endpoint=None, view_func=None, **options):
+        if rule not in self._skip_rules:
+            super(BlueprintSetupStateSkip, self).add_url_rule(rule, endpoint, view_func, **options)
+
+class BlueprintSkip(Blueprint):
+    def __init__(self, *args, **kwargs):
+        super(BlueprintSkip, self).__init__(*args, **kwargs)
+
+    def make_setup_state(self, app, options, first_registration=False):
+        skip_rules = options.pop("skip_rules", [])
+        return BlueprintSetupStateSkip(self, app, options, first_registration, skip_rules)