`rucio --help` fails on misconfigured sites
Motivation
----------
Even on a bad installation or typos in the configuration system, `rucio --help` ought to work.

The current version of `rucio` imports the `DownloadClient` at the module level.  If there is an error in the `rucio.cfg` that causes the `DownloadClient` constructor to throw an exception, you can't get to `--help`.

Example:
```
$ rucio --help 
Traceback (most recent call last):
  File "/home/cse496/bbockelm/software/rucio-doma/bin/rucio", line 63, in <module>
    from rucio.client.downloadclient import DownloadClient
  File "/home/cse496/bbockelm/projects/rucio/lib/rucio/client/downloadclient.py", line 34, in <module>
    client = Client()
... stack trace goes on for miles ...
  File "/home/cse496/bbockelm/software/rucio-doma/lib/python2.7/site-packages/requests/adapters.py", line 412, in send
    self.cert_verify(conn, request.url, verify, cert)
  File "/home/cse496/bbockelm/software/rucio-doma/lib/python2.7/site-packages/requests/adapters.py", line 227, in cert_verify
    "invalid path: {0}".format(cert_loc))
IOError: Could not find a suitable TLS CA certificate bundle, invalid path: /etc/ca.crt
```

In my case, the problem was caused by an invalid path in `rucio.cfg` for the CA bundle.

Modification
------------
Failures to create the clients should be swallowed until they are actually used.  Potentially we shouldn't even import the clients until needed.
diff --git a/lib/rucio/common/config.py b/lib/rucio/common/config.py
index 19cadfc5a..d3afacd04 100644
--- a/lib/rucio/common/config.py
+++ b/lib/rucio/common/config.py
@@ -54,7 +54,7 @@ def config_get(section, option, raise_exception=True, default=None, clean_cached
     global __CONFIG
     try:
         return get_config().get(section, option)
-    except (ConfigParser.NoOptionError, ConfigParser.NoSectionError) as err:
+    except (ConfigParser.NoOptionError, ConfigParser.NoSectionError, RuntimeError) as err:
         if raise_exception and default is None:
             raise err
         if clean_cached:
['lib/rucio/common/config.py']