shadowsocks-libev: move plugin options to server section

shadowsocks-libev: move plugin options to server section

Plugin options are properties of shadowsocks deployment as a whole,
including both server and each client components.  Multiple client
instances accessing the same server will need to share the same plugin
settings

With this change, plugin options will need to specified to "server" and
"ss-server" section, not to each component section.

Fixes: c19e949 ("shadowsocks-libev: add plugin options support")
Reference: https://github.com/openwrt/packages/issues/8903#issuecomment-489674137
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>