Describe the bug
The OpenVPN parameter tls_cipher should be an array and not a string. It should render to a UCI list item and not to an option.

Steps To Reproduce
Currently the parameter tls_cipher is a string and renders to an option. This results in the client not working when a specific cipher string is configured:

option tls_cipher 'TLS-DHE-RSA-WITH-AES-256-CBC-SHA:@SECLEVEL=0' does not work.
list tls_cipher 'TLS-DHE-RSA-WITH-AES-256-CBC-SHA:@SECLEVEL=0' works.

Expected behavior
The OpenVPN parameter tls_cipher should be an array and not a string. It should render to a UCI list item and not to an option.
In the LuCI OpenVPN app the parameter tls_cipher is implemented by a dynamic list: https://github.com/openwrt/luci/blob/master/applications/luci-app-openvpn/luasrc/model/cbi/openvpn-advanced.lua#L687
This also renders to UCI list: https://github.com/openwrt/luci/wiki/CBI#class-dynamiclist-option-title-description