[tool] Add dev.py --isolate option

This creates a environment for tests that is isolated from the current
environment (as long as there are no .egg-link to e.g. site-packages).

dev.py

@@ -222,18 +222,40 @@ def main(args):
     except ImportError:
         install_coverage()
 
+    ######################################################################
+    bin_buildout = os.path.join('bin', 'buildout')
+
+    version_args = [
+        'versions:setuptools=' + (args.setuptools_version or ''),
+        'versions:pip=' + (args.pip_version or ''),
+    ]
+
+    if args.isolate:
+        print('')
+        print('Bootstrap isolated buildout')
+        print('')
+        if subprocess.call(
+            [bin_buildout, 'buildout:extra-paths='] +
+            version_args +
+            ['bootstrap']
+            ):
+            raise RuntimeError("failed to bootstrap isolated buildout")
+
     ######################################################################
     print('')
     print('Run buildout')
     print('')
-    bin_buildout = os.path.join('bin', 'buildout')
 
     if sys.platform.startswith('java'):
         # Jython needs the script to be called twice via sys.executable
-        assert subprocess.Popen([sys.executable, bin_buildout, '-N']).wait() == 0
+        assert subprocess.Popen(
+            [sys.executable, bin_buildout, '-N'] + version_args
+            ).wait() == 0
 
     sys.stdout.flush()
-    sys.exit(subprocess.Popen(bin_buildout).wait())
+    sys.exit(subprocess.Popen(
+        [bin_buildout, '-N'] + version_args
+        ).wait())
 
 def parse_args():
     import argparse
@@ -247,6 +269,9 @@ def parse_args():
     parser.add_argument('--no-clean', 
         help='not used in the code, find out if still needed in Makefile',
                         action='store_const', const='NO_CLEAN')
+    parser.add_argument('--isolate',
+        help='isolate from environment',
+                        action='store_true', default=False)
 
     args = parser.parse_args()
     return args