Set DeleteOnTermination for in BlockDeviceMappings to false in case of volumes with RootDiskType

[dergeberl]

What this PR does / why we need it:

In case of a RootDiskType is specified the volume will be created before creating the server. Also while deletion the server gets deleted first and than also the volume is getting deleted.
In the BlockDeviceMapping currently we set DeleteOnTermination: true this is wrong because the volume gets deleted by the mcm.

Special notes for your reviewer:

Release note:

Set DeleteOnTermination for in BlockDeviceMappings to false in case of volumes with RootDiskType.

[gardener-robot]

@dergeberl Thank you for your contribution.

[gardener-robot-ci-3]

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[kon-angelo]

/lgtm

[kon-angelo]

@dergeberl I understand that since we already have code to delete the volume, that setting this flag is redundant and this change is fine. Out of curiosity, did you encounter and issue with that so far, or just removing the redundancy ?

[dergeberl]

Out of curiosity, did you encounter and issue with that so far, or just removing the redundancy ?

We had rare problems while deletion of a volume. During that we found this redundancy. I am not sure if this was the root cause of that problem but I wanted to change it to have clear responsibilities who deleted the volume.