Use a timeout when performing requests to remote urls

[capooti]

I have the feeling that some celery tasks hangs indefinitely waiting for an http response from servers.
We should set timeouts for all tasks which perform requests to remote servers.

Two possible approaches:

1. use timeout in celery task (easier)
2. use timeout with requests, arcrest and owslib. I am not sure though if arcrest and owslib have the option to set a timeout

[capooti]

@thaorell can you look into this? Thanks

[capooti]

we should have it as a django settings, so we can change if needed. thanks