cachecontrol: Explicitly pre-install the cachecontrol and its dependencies

   It prevents things skip offline control.

component/python-cachecontrol/buildout.cfg

+[buildout]
+
+[python-cachecontrol]
+recipe = zc.recipe.egg:custom
+egg = cachecontrol
+setup-eggs =
+  pbr
+  lockfile

component/slapos/buildout.cfg

@@ -77,6 +77,7 @@ eggs =
   ${lxml-python:egg}
   ${python-cffi:egg}
   ${python-PyYAML:egg}
+  ${python-cachecontrol:egg}
   ${python-cliff:egg}
   ${python-cryptography:egg}
   pyOpenSSL

stack/slapos.cfg

@@ -27,6 +27,7 @@ extends =
   ../component/python-2.7/buildout.cfg
   ../component/python-cffi/buildout.cfg
   ../component/python-cliff/buildout.cfg
+  ../component/python-cachecontrol/buildout.cfg
   ../component/python-cryptography/buildout.cfg
   ../component/python-PyYAML/buildout.cfg
 
@@ -91,6 +92,7 @@ eggs =
   ${lxml-python:egg}
   ${python-cffi:egg}
   ${python-PyYAML:egg}
+  ${python-cachecontrol:egg}
   ${python-cliff:egg}
   ${python-cryptography:egg}
   pyOpenSSL