ovsdb-doc: generate vswitch.[pic|gv] files only if dot tool is available

ovsdb-doc: generate vswitch.[pic|gv] files only if dot tool is available

These are auto-generated files, so it would be better not to keep them
inside Open vSwitch repository.

Behaviour before this patch was that if dot tool was not present on
the system, then ovs-vswitchd.conf.db.5 would have used pre-generated
vswitch.pic file that was already checked in the git repository. After
this patch ovs-vswitchd.conf.db.5 will simply not have a dot diagram,
if dot was not present at the time when Open vSwitch was built.

Signed-off-by: Ansis Atteka <aatteka@nicira.com>