dot2pic: Be less picky parsing "dot" output.

dot2pic: Be less picky parsing "dot" output.

Some versions of "dot" put two spaces after the "node" keyword instead of
one, which didn't match the regular expression used in dot2pic.  This
commit changes dot2pic not to care about the number of spaces in "node" and
"graph" lines.  (The "graph" lines weren't actually a problem but I don't
see a reason to be picky about them either.)

Different versions of "dot" still produce different output for the same
input, but I don't see how to avoid that.

Reported-by: Justin Pettit <jpettit@nicira.com>