ofp-parse: Correctly parse actions= in group bucket

Make the parser compatible with how we print group buckets and allow
actions= to be specified, e.g.:

 group_id=1,type=all,bucket=actions=output:2,bucket=actions=output:3

Also, better document the supported syntax in the manual page and
cover both syntax formats in the unit test.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>