VM allocation on Primary Storage

##### ISSUE TYPE

* Enhancement Request

##### COMPONENT NAME

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Primary Storage
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##### CLOUDSTACK VERSION

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N.R.
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##### CONFIGURATION

##### OS / ENVIRONMENT

##### SUMMARY

There should be better management regarding resource allocation on primary storages.
This could be done by allowing the user to be able to choose the disk offering when creating a VM via template via UI (as already happens when creating a VM via API).

Also the parameter "Preferred storage pool" (preferred.storage.pool) (The UUID of preferred storage pool for allocation.) should be configurable in the Cluster or Zone settings so the admin could always choose the preferred PS for every zone. This could be very useful also during storage migrations.

##### STEPS TO REPRODUCE

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##### EXPECTED RESULTS

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##### ACTUAL RESULTS

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