Add 'tail' command to AppScale

Tailing logs can often be an unnecessarily repetitive process (finding IP addresses, logging in, constructing the tail command, etc). This pull request remedies this problem by adding `appscale tail id regex`, where `id` indicates the index of the node to tail from, and `regex` indicates the regular expression that should be used to tail logs from `/var/log/appscale/`.

If `id` and `regex` are not provided, they default to `0` and `c*`, tailing logs from the AppController on the first node (which is typically the head node).