Horizon changes to support subcloud-group based orchestration

[Jessica Castelino]

Brief Description
-----------------
With the introduction of subcloud-group based orchestration, the attributes "subcloud_apply_type" and "max_parallel_subclouds" have become optional. Right now, there is an asterisk next to these parameters on Horizon which makes it mandatory for the user to provide these values while creating a patch strategy or an orchestration strategy.

Changes have to be made to Horizon to make these parameters optional so that there is consistency between the CLI and GUI.

Severity
--------
Minor

Steps to Reproduce
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Open patch strategy/upgrade strategy creation forms on Horizon

Expected Behavior
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The attributes "subcloud_apply_type" and "max_parallel_subclouds" must be optional

Actual Behavior
----------------
The attributes "subcloud_apply_type" and "max_parallel_subclouds" aren't optional

Reproducibility
---------------
100% reproducible

[Ghada Khalil (gkhalil)]

low priority / minor cleanup