docs: Fix overlapping 'weak' edges in ovs-vswitchd.conf.db.5.

docs: Fix overlapping 'weak' edges in ovs-vswitchd.conf.db.5.

Multiple weak edges between nodes at the same rank overlaps with each other in
a dot/graphviz diagram. The vswitchd.pic used in ovs-vswitchd.conf.db.5 suffers
this problem.

Removing "constraint=false" allows graphviz to rank the nodes using the weak
edages as well so that the nodes at the ends of a weak edge won't be at the
same rank and allows mutlple 'weak' edges to be visible.

Signed-off-by: Shu Shen <shu.shen@radisys.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>