[docs] Add installation instruction

[docs] Add installation instruction

For now, only script-based installation is described, since it's
the most flexible and easy to use in various environments.

Script-based installation also assumes the jumpserver/cluster nodes
have preinstalled operating systems, simplifying the configuration
necessary for more complete installation methods that also cover
VM/baremetal provisioning (e.g. OPNFV Compass/Fuel, which require
a lot more input, especially for baremetal deployments).

This can later be extended with Compass/Fuel/Airship/Heat specific
instructions.

JIRA: IEC-9

Change-Id: I34d274ed03b70c439c60db02df1e0e55c2c11b34
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>