Allow to use buildout found in path.

If buildout binary is not passed the next step is try to import zc.buildout
before falling back to internal bootstrap.

Thank to this it will be possible to use system or buildout provided buildout
with all related dependencies (like slapos.libnetworkcache).

slapos/grid/utils.py

@@ -228,14 +228,26 @@ def bootstrapBuildout(path, buildout=None,
   gid = stat_info.st_gid
 
   invocation_list = [sys.executable, '-S']
+  kw = dict()
   if buildout is not None:
     invocation_list.append(buildout)
+    invocation_list.extend(additional_buildout_parametr_list)
   else:
+    try:
+      import zc.buildout
+    except ImportError:
       logger.warning('Using old style bootstrap of included bootstrap file. '
         'Consider setting buildout binary location.')
       invocation_list.append(pkg_resources.resource_filename(__name__,
         'zc.buildout-bootstap.py'))
       invocation_list.extend(additional_buildout_parametr_list)
+    else:
+      # buildout is importable, so use this one
+      invocation_list.extend(["-c", "import sys ; sys.path=" + str(sys.path) +
+        " ; import zc.buildout.buildout ; sys.argv[1:1]=" + \
+        repr(additional_buildout_parametr_list + ['bootstrap']) + " ; "
+        "zc.buildout.buildout.main()"])
+
   if buildout is not None:
     invocation_list.append('bootstrap')
   try:
@@ -243,7 +255,6 @@ def bootstrapBuildout(path, buildout=None,
     logger.debug('Set umask from %03o to %03o' % (umask, SAFE_UMASK))
     logger.debug('Invoking: %r in directory %r' % (' '.join(invocation_list),
       path))
-    kw = dict()
     if not console:
       kw.update(stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
     process_handler = SlapPopen(invocation_list,