Catch DistributionNotFound when pycrypto is absent (#15731)

* Catch DistributionNotFound when pycrypto is absent

On Solaris 11, module `pkg_resources` throws `DistributionNotFound` on import if `cryptography` is installed but `pycrypto` is not.  This change causes that situation to be handled gracefully.

I'm not using Paramiko or Vault, so I my understanding is that I don't
need `pycrpto`.  I could install `pycrypto` to make the error go away, but:
- The latest released version of `pycrypto` doesn't build cleanly on Solaris (https://github.com/dlitz/pycrypto/issues/184).
- Solaris includes an old version of GMP that triggers warnings every time Ansible runs (https://github.com/ansible/ansible/issues/6941).  I notice that I can silence these warnings with `system_warnings` in `ansible.cfg`, but not installing `pycrypto` seems like a safer solution.

* Ignore only `pkg_resources.DistributionNotFound`, not other exceptions.

lib/ansible/parsing/vault/__init__.py

@@ -70,6 +70,11 @@ try:
     HAS_PBKDF2HMAC = True
 except ImportError:
     pass
+except Exception as e:
+    if e.__module__ == 'pkg_resources' and e.__class__.__name__ == 'DistributionNotFound':
+        pass
+    else:
+        raise
 
 from ansible.compat.six import PY3
 from ansible.utils.unicode import to_unicode, to_bytes