Add scenario modyfing existring group in the group table

This scenario verifies that a group works correctly after it is modified.
(Motivated by #228).
Fix modifying existing group in a group table

Now ofp_buckets are translated to link_buckets
and linc_us4_gruops:apply_bucket/2 works as expected.
(Motivated by #228).
Merge pull request #230 from FlowForwarding/group-table-modification-crash

Group table modification crash