Replace hardcoded openvpn ping and ping-restart with server provided values.

We need to replace the hardcoded openvpn settings with the values stored in @eip-service.json@.

Current code: https://github.com/leapcode/bitmask_client/blob/develop/src/leap/bitmask/services/eip/vpnlauncher.py#L249

args += [
    '--ping', '10',
    '--ping-restart', '30']

We need to use the 'keepalive' settings given by the provider (implemented by #4127). Look for @"keepalive": "10 30"@ in @eip-service.json@.

(from redmine: created on 2013-10-10)