luci-app-openvpn: add possibility to set param "compress" without algorithm

In some situations you need to set the compress param without an
algorithm. Compression will be turned off, but the packet framing for
compression will still be enabled, allowing a different setting to be
pushed later.

As it is not possible to have options with optional values at the
moment, I've introduced a pseudo value "frames_only" which will be
removed in the init script.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>