Fix: Promenade Exceptions docs rendering on RTD

Readthedocs failed to render Promenade exceptions with error:
> WARNING: autodoc: failed to import exception 'xxx' from module
> 'promenade'; the following exception was raised: No module
> named 'falcon'

Trying to add Promenade requirements to the installed requirements list,
so that Readthedocs has all modules, including those needed for the
Promenade itself.

Unify docs building by utilizing Zuul docs-on-readthedocs template job.

Cosmetic readability changes:
1. combined all Makefile .PHONY targets into one
2. merged multiple LABEL instructions in Dockerfile into one

Change-Id: I731ee3426a631fa765f13ba7091dcb4b9ebd0353

.zuul.yaml

@@ -11,12 +11,16 @@
 # limitations under the License.
 
 - project:
+    templates:
+      - docs-on-readthedocs
+    vars:
+      rtd_webhook_id: '38575'
+      rtd_project_name: 'airship-promenade'
     check:
       jobs:
         - openstack-tox-pep8
         - airship-promenade-lint-ws
         - airship-promenade-docker-build-gate
-        - airship-promenade-doc-build
         - airship-promenade-chart-build-gate
         - airship-promenade-chart-build-latest-htk
         - airship-promenade-unit-py35
@@ -27,7 +31,6 @@
         - openstack-tox-pep8
         - airship-promenade-lint-ws
         - airship-promenade-docker-build-gate
-        - airship-promenade-doc-build
         - airship-promenade-chart-build-gate
         - airship-promenade-unit-py35
         - airship-promenade-genesis-gate
@@ -88,19 +91,6 @@
     files:
       - ^.*\.py$
 
-- job:
-    name: airship-promenade-doc-build
-    description: |
-      Locally build the documentation to check for errors
-    run: tools/zuul/playbooks/doc-build.yaml
-    timeout: 300
-    nodeset: airship-promenade-single-node
-    irrelevant-files:
-      - ^charts/.*$
-      - ^etc/.*$
-      - ^tests/.*$
-      - ^tools/.*$
-
 - job:
     name: airship-promenade-linter
     run: tools/zuul/playbooks/zuul-linter.yaml