Get riseup stuff out of menshen

Making menshen provider independent. Currently, the code contains some riseup stuff:

pkg/api/providers.go
27:var defaultVPNWebCert = "https://api.black.riseup.net/3/cert"
29:var riseupCA = `-----BEGIN CERTIFICATE-----

pkg/genconfig/serialize.go
90:var riseupCA = `-----BEGIN CERTIFICATE-----

pkg/api/genconfig.go
60: "riseup.ovpn",

There are still some RiseUp things in texts (like docs/help, e.g. pkg/help/ios.go).

Proposal

• support for command line argument + env variable: defaultVPNWebCert
• support for command line argument + env variable: vpnCaFile (content: filename, not the ca itself)
  • comment says: TODO get the CA from the service too - how does this work?
• riseup.ovpn => leap.ovpn

I can fix the issue if you tell me how :)