Avoid a full install in tox environments that do not need it

Usage of usedevelop=True results in all requirements.txt dependencies
always installed (without constraints) even when requirements.txt
is not explicitly added. Override usedevelop for these environments.

Add requirements.txt to the 'docs' environment since it imports
the whole ironic in the process.

Change-Id: Ibab7d9cff1bd6eccc022873d1c5f59e27e54abaf

tox.ini

@@ -31,6 +31,7 @@ deps = {[testenv]deps}
 commands = {toxinidir}/tools/states_to_dot.py -f {toxinidir}/doc/source/images/states.svg --format svg
 
 [testenv:pep8]
+usedevelop = False
 deps=
     hacking>=3.1.0,<4.0.0 # Apache-2.0
     doc8>=0.6.0 # Apache-2.0
@@ -76,8 +77,10 @@ commands =
 commands = oslo_debug_helper -t ironic/tests/unit {posargs}
 
 [testenv:docs]
+# NOTE(dtantsur): documentation building process requires importing ironic
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -b html -W doc/source doc/build/html
 
@@ -90,6 +93,7 @@ commands =
 
 
 [testenv:api-ref]
+usedevelop = False
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -99,6 +103,7 @@ commands =
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
 [testenv:releasenotes]
+usedevelop = False
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -162,5 +167,6 @@ deps =
   -r{toxinidir}/requirements.txt
 
 [testenv:bandit]
+usedevelop = False
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r ironic -x tests -n5 -ll -c tools/bandit.yml