python2-7: use intermediate install section

because we need to export `:executable`, we guessed the installation
location. Instead, introduce a new section to install.

`python2.7` section just expose the options and will be available only
after build.

component/python-2.7/buildout.cfg

@@ -27,17 +27,16 @@ arguments = sys.argv[1:] + ["bootstrap"]
 python = python2.7
 
 [python2.7]
+version = 2.7
+executable = ${:location}/bin/python${:version}
+location = ${python2.7-install:location}
+
+[python2.7-install]
 recipe = slapos.recipe.cmmi
 package_version = 2.7.14
 package_version_suffix =
 md5sum = 1f6db41ad91d9eb0a6f0c769b8613c5b
 
-# This is actually the default setting for prefix, but we can't use it in
-# other settings in this part if we don't set it explicitly here.
-prefix = ${buildout:parts-directory}/${:_buildout_section_name_}
-version = 2.7
-executable = ${:prefix}/bin/python${:version}
-
 patch-options = -p1
 patches =
   ${:_profile_base_location_}/fix_compiler_module_issue_20613.patch#94443a77f903e9de880a029967fa6aa7