Ignore empty/comment lines in OS variants file

Per a conversation on ganeti@googlegroups.com:

  - gnt-os diagnose ; gnt-os list take in consideration blank lines in
   /etc/ganeti/instance-image/variants.list that could be confusing.

Let's fix this and also let's ignore comment lines.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>