Before this commit, provisioning was done by executing provision.py,
which printed the log directly to stdout, making debugging harder.
This commit creates a wrapper bash script 'provision' in tools, which
calls 'zulip/scripts/tools/provision_vm.py' (the new location of
provision.py) and prints all the output to
'zulip/var/log/zulip/zulip_provision.log' via 'tee'.
Travis tests and docs have been modified accordingly.
Robert Hönig