fix(openconnect): let openconnect use its config

[david-jointech]

• Openconnect will now use the custom config provided to Vopono.
• Openconnect is now able to authenticate with the provided password
• It's not possible to provide a username via Vopono to Openconnect. This is due to there not being a simple optional way to provide it and the username clashing with the one provided in the openconnect-config.
• Openconnect will use the optionally provided server, or the server from its custom config. If both are provided openconnect will fail.

An example minimal openconnect.conf which can be provided as a custom config:
with server:

server = vpn.example.com
user = user

without server:

user = user

Generally you can add all command-line-arguments from openconnect. E.g. my tested VPN-config would look like this:

server = vpn.example.com
user = user
authgroup = group
no-dtls

from this oppenconnect-command: openconnect --user user --authgroup group --no-dtls --passwd-on-stdin vpn.example.com. Note, that passwd-onstdin is always added in vopono and shouldn't be set in the config.

Fixes #39

[jamesmcm]

Thanks this looks great, I can't test it directly without having an OpenConnect VPN anymore, but it looks good.