openvpn: quote parameters to --push in openvpn config file

OpenVPN requires arguments to --push to be enclosed in double quotes.
One set of quotes is stripped when the UCI config is parsed.
Change append_params() of openvpn.init to enclose push parameters in
double quotes.

Unquoted push parameters do not cause errors in OpenVPN 2.3,
but OpenVPN 2.4 fails to start with unquoted push parameters.

Fixes: FS#290.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>