nuntium should re-try activating a context if there is no proxy

After latest nuntium changes a proxy is always specified as part of the ofono properties, so if there is no proxy it should be considered an activation error and nuntium should try to re-open the context up to the usual limit of 3 times. See attached logs for a case when this happens.

This problems seem to happen occasionally when we need to do a DNS query to find out the proxy IP address. The c-ares logic in ofono should be analysed too as maybe it is not retrying hard enough the DNS query.