`import rucio.core.rule` fails with missing section
The work in #339 caused a regression if the `permission` section does not exist.  From the logs:

```
mod_wsgi (pid=18803): Exception occurred processing WSGI script '/usr/lib/python2.7/site-packages/rucio/web/rest/did.py'.
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/rucio/web/rest/did.py", line 22, in <module>
    from rucio.api.did import (add_did, add_dids, list_content, list_content_history,
  File "/usr/lib/python2.7/site-packages/rucio/api/did.py", line 19, in <module>
    from rucio.core import did, naming_convention, meta as meta_core
  File "/usr/lib/python2.7/site-packages/rucio/core/did.py", line 35, in <module>
    import rucio.core.rule
  File "/usr/lib/python2.7/site-packages/rucio/core/rule.py", line 2888, in <module>
    def archive_localgroupdisk_datasets(scope, name, session=None):
  File "/usr/lib/python2.7/site-packages/rucio/common/policy.py", line 78, in policy_filter
    policy = get_policy()
  File "/usr/lib/python2.7/site-packages/rucio/common/policy.py", line 40, in get_policy
    policy = config_get('permission', 'policy')
  File "/usr/lib/python2.7/site-packages/rucio/common/config.py", line 30, in config_get
    return __CONFIG.get(section, option)
  File "/usr/lib64/python2.7/ConfigParser.py", line 607, in get
    raise NoSectionError(section)
NoSectionError: No section: 'permission'
```

Seems that this exception type is simply not caught.
diff --git a/lib/rucio/common/policy.py b/lib/rucio/common/policy.py
index b0c3efc7e..62403709c 100644
--- a/lib/rucio/common/policy.py
+++ b/lib/rucio/common/policy.py
@@ -38,7 +38,7 @@ def get_policy():
     if isinstance(policy, NoValue):
         try:
             policy = config_get('permission', 'policy')
-        except NoOptionError:
+        except (NoOptionError, NoSectionError):
             policy = 'atlas'
         REGION.set('policy', policy)
     return policy
['lib/rucio/common/policy.py']