plugin/core.c: Add distribution tree dumping to log

Distribution tree is dumped to log when it
is changed (after SUBNET_UP event) and when
"distrib_tree_level" option is enabled in core
configuration file.

"distrib_tree_level" option is a bitmap of
distribution tree layers that should be dumped
to log:
	bit 0 : core layer
	bit 1 : distribution layer
	bit 2 : access layer
	bit 3 : consumer (acm) layer

* this option is disabled (value 0) by default

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>