drop_privileges should also set HOME

E.g. if HOME is not set, swift-proxy will create the
keystone_signing file not in HOME but in /root.
This is because the swift user doesn't have a shell
in /etc/passwd and so it doesn't set environment variables
when impersonating.

Change-Id: I3013007e0dadf6ddccc176e142b7c78c5d63a351

swift/common/utils.py

@@ -756,6 +756,7 @@ def drop_privileges(user):
         os.setgroups([])
     os.setgid(user[3])
     os.setuid(user[2])
+    os.environ['HOME'] = user[5]
     try:
         os.setsid()
     except OSError:

test/unit/common/test_utils.py

@@ -632,6 +632,8 @@ log_name = %(yarr)s'''
         utils.drop_privileges(user)
         for func in required_func_calls:
             self.assert_(utils.os.called_funcs[func])
+        import pwd
+        self.assertEquals(pwd.getpwnam(user)[5], utils.os.environ['HOME'])
 
         # reset; test same args, OSError trying to get session leader
         utils.os = MockOs(called_funcs=required_func_calls,