Reduce min data_disk_count to 4 in IO perf test.

[SRIKKANTH]

Having min data_disk_count =16 in these testcases is blocking tests on smaller sizes.

[LiliDeng]

@johnsongeorge-w please review, thanks.

[johnsongeorge-w]

Set the default for disk count and min iops as 16 and 5000 respectively in the default runbook.

[SRIKKANTH]

Set the default for disk count and min iops as 16 and 5000 respectively.
@johnsongeorge-w This is what the default in the test case which is blocking us the test from running on smaller tests.
I am reducing the min disc count to 4 which is the minimum number of discs supported any azure VM SKU.

[johnsongeorge-w]

Set the default for disk count and min iops as 16 and 5000 respectively.
@johnsongeorge-w This is what the default in the test case which is blocking us the test from running on smaller tests.
I am reducing the min disc count to 4 which is the minimum number of discs supported any azure VM SKU.

I'm talking about the defaults in runbook

[SRIKKANTH]

Set the default for disk count and min iops as 16 and 5000 respectively.
@johnsongeorge-w This is what the default in the test case which is blocking us the test from running on smaller tests.
I am reducing the min disc count to 4 which is the minimum number of discs supported any azure VM SKU.

I'm talking about the defaults in runbook
The default runbook ("runbook\partial\platform_azure.yml") has values data_disk_iops=0, data_disk_count=0.
Changing these values will result in creation of each test VM with that config.
I don't think its a good idea.