ovs-controller: Make --with-flows read the file only once, at startup.

ovs-controller: Make --with-flows read the file only once, at startup.

A couple of people have reported that ovs-controller --with-flows is
confusing.  This seems to be because it doesn't read the file with the
flows until the first connection from a switch.  Then, if the file has a
syntax error, it exits.

This commit changes the behavior so that it reads the file immediately at
startup instead.