[http] always use a timeout by default
[http] always use a timeout by default

Currently, in case of an unreachable cluster, multiple commands would
hang indefinitely (eg. "dcos node", "dcos package list", etc.).

This makes the default timeout for http.request() to be 5 seconds.

dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli