tools: Remove cloudmonkey, add info in INSTALL.md

After some discussion on the dev ML[1], we decided to move tools/cli which
contained cloudmonkey to a new git repository [2]. We did that by retaining
its history. In this commit, we remove tools/cli and add information on where
to find cloudmonkey. This is help us speed up cloudmonkey's development and
releases, now that with ApiDiscovery it's completely independent of any other
CloudStack modules.

[1] http://markmail.org/message/tjlr753xfhpw4uk4
[2] https://git-wip-us.apache.org/repos/asf?p=cloudstack-cloudmonkey.git

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>