Increase default timeout of Exec resources to 900 seconds

Puppet's default value of 300 may be appropriate for servers, but it isn't
always appropriate for users on slow connections. It's better to have 'apt-get
update' take longer than fail outright.

We could set the timeout parameter to 0, which disables timeouts, but I think e
do want hung Execs to *eventually* fail.

Change-Id: I16eb312b19ca0d93c858d2e84bafbd36c8053a4b