ovs-ctl: Prepend OVS binary directories to PATH.

ovs-ctl: Prepend OVS binary directories to PATH.

The inclusion of LSB functions in ovs-lib resets $PATH to
system's default. Then ovs-ctl appends missing directories
including the OVS default ones $sbindir and $bindir.

The problem is that the wrong binaries can be used if they
are available in the system's default locations because of
the PATH wrong order. The same issue happens if one changes
$OVS_BINDIR or $OVS_SBINDIR variables.

The solution is to prepend those directories if they are not
already in PATH.

Reported-by: Mark Gray <mark.d.gray@redhat.com>
Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Mark Gray <mark.d.gray@redhat.com>
Acked-by: Paolo Valerio <pvalerio@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>