luci-app-acme: Rename keylength to key_type

The keylength option was renamed to key_type and values changed.
See https://github.com/openwrt/packages/pull/21079/commits/6d61014e51266f1cb083d9f31491f9c5fb73eeb0
Make it optional to let acme.sh to decide (currently ec256)

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>