[octane] Octane doesn't handle multinode (non-HA) environments in from 6.0

[Roman Sokolkov]

Octane's upgrade-env migrates cluster mode AS IS. For example, 6.0 had also "multinode" (non HA) mode, which is not available in later versions.

From 6.1 there is ONLY correct mode "ha_compact".

Steps to reproduce:
- Install 6.0 w/o HA
- upgrade Fuel master to 9.1
- Install octane and execute "upgrade-env"

Expected result:
Cluster will have correct mode "ha_compact"

Actual result:
Cluster has mode "multinode" and this breaks deployment

[Ilya Kharin (akscram)]

This is happening because cluster-upgrade extension creates a new environment with exactly the same parameters, such as mode and net_provider, as an original one has. It seems as a lag in validation of users' data and operator possibilities. Several simple improvements can be made here:

 * re-use validation of the mode parameter for for a new cluster to prevent of using incorrect modes for releases [1]

 * add an additional optional parameter "mode" to provide ability to override the original one [2]

[1] https://github.com/openstack/fuel-web/blob/master/nailgun/nailgun/api/v1/validators/cluster.py#L122-L132
[2] https://github.com/openstack/fuel-nailgun-extension-cluster-upgrade/blob/master/cluster_upgrade/validators.py#L32-L35