[feature] Allow to use their own CA certificate file

[gobiursus]

Is your feature request related to a problem? Please describe.
I can't make settings in .env file to let openwisp to use my own CA.

Describe the solution you'd like
Allow openwisp to use user's CA

[nemesifier]

@gobiursus are you talking about the CA for OpenVPN?

[gobiursus]

@gobiursus are you talking about the CA for OpenVPN?

I want API, OpenVPN, dashboard and everything use the same self-signed CA and my self-signed SSL keypair signed by the CA.