dir-spec doesn't specify ntor-onion-key
Section 2.1. of dir-spec does not specify descriptor 'ntor-onion-key' which is a part of /stem/test/data/cached-descriptors file which gives an "Unrecognized descriptor content" error message causing a failure in test_cached_descriptor
```
family $7E5F5B17CAA9BEF929F57D97EEFE305A2089E901
hidden-service-dir
contact +1 303-265-9283, rdump@OFTC
ntor-onion-key Od2Sj3UXFyDjwESLXk6fhatqW9z/oBL/vAKJ+tbDqUU=
reject 0.0.0.0/8:*
reject 169.254.0.0/16:*
reject 127.0.0.0/8:*
```
**Trac**:
**Username**: sonu
issue