BackUp Host Selection Algorithm

Summary
=======
There should be support for BackUp Host Selection Algorithm to select the most appropriate host for BackUp instead of selecting BackUp Host Randomly whenever multiple Hosts are available. There should be some parameters on the basis of which a host can be preferred for the backup of volume.

Motivation
========
There are certain parameters and qualities of each host. Some hosts can have better read/write speeds, some can have bigger free space available, some can have other parameters which make them better choice for certain kind of volumes. Therefore, if we can implement a backup host selection algorithm to chose the most appropriate host out of the available hosts, then it would result in better quality of services and efficient use of resources.

Description
=========
Whenever multiple backup hosts are available, it is wise to choose the most appropriate hosts for backup, which can result in efficient execution of the use case. There must be some algorithm to select the most appropriate host for BackUp instead of selecting BackUp Host Randomly whenever multiple Hosts are available. There should be some parameters on the basis of which a host can be preferred for the backup of volume. User can provide the parameters as argument or according to the current volume, then the appropriate backup host should be chosen whenever multiple backup hosts are available.

UX
===
- There is no need to make any changes in user interface, same interface can be used, however the execution or the functionality of choosing the backup host need to be changed.

Testing
======
Proper Tests would be added to check the system's behaviour after adding this feature.

Outside Dependencies
==================
Functionality must be added to cinder cli, openstack cli and horizon, after the implementation of this feature.

Requirements Update Required
========================
Update requirement versions for each the packages ie. cinder-cli and openstack-cli.

Doc Impact
=========
Cinder documentation will be updated in order to expose to the user the new features that will be introduced. The configuration details can be added in the cinder documentation about this backup host selection method.