vswitchd: Allow user to directly specify sFlow agent address.

vswitchd: Allow user to directly specify sFlow agent address.

At least for testing purposes, and perhaps in production, it is useful to
be able to fix the agent IP address directly, rather that indirecting it
through a device name or the routing table.

This commit uses this feature to fix the agent IP address used in the unit
tests.  This will be particularly useful in an upcoming commit that
disables the use of the system routing table in the unit tests, to make
the tests' results independent of the host's routes.

CC: Neil McKee <neil.mckee@inmon.com>
Tested-By: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>