Refactor azurerm_network_watcher_flow_log and add supports for location and tags

[magodo]

This refactors the azurerm_network_watcher_flow_log to using its own separate endpoint - rather than using the configureFlowLog endpoint of the Network Watcher (more details here)

This PR is implemented in a way to avoid breaking changes on the existing instances and ensure backward compatiblility (locally tested).

Besides, I've modified the test cases to make them sequential - the prior implementation actually are still running in parallel at the end.

Fixes: #11587

Test Result

💤 TF_ACC=1 go test -v -timeout=8h ./azurerm/internal/services/network -run="TestAccNetworkWatcherFlowLog_"
2021/05/12 14:25:23 [DEBUG] not using binary driver name, it's no longer needed
2021/05/12 14:25:23 [DEBUG] not using binary driver name, it's no longer needed
=== RUN   TestAccNetworkWatcherFlowLog_basic
--- PASS: TestAccNetworkWatcherFlowLog_basic (228.54s)
=== RUN   TestAccNetworkWatcherFlowLog_disabled
--- PASS: TestAccNetworkWatcherFlowLog_disabled (228.47s)
=== RUN   TestAccNetworkWatcherFlowLog_reenabled
--- PASS: TestAccNetworkWatcherFlowLog_reenabled (298.10s)
=== RUN   TestAccNetworkWatcherFlowLog_retentionPolicy
--- PASS: TestAccNetworkWatcherFlowLog_retentionPolicy (323.25s)
=== RUN   TestAccNetworkWatcherFlowLog_updateStorageAccount
--- PASS: TestAccNetworkWatcherFlowLog_updateStorageAccount (359.24s)
=== RUN   TestAccNetworkWatcherFlowLog_trafficAnalytics
--- PASS: TestAccNetworkWatcherFlowLog_trafficAnalytics (566.27s)
=== RUN   TestAccNetworkWatcherFlowLog_version
--- PASS: TestAccNetworkWatcherFlowLog_version (371.40s)
=== RUN   TestAccNetworkWatcherFlowLog_location
--- PASS: TestAccNetworkWatcherFlowLog_location (234.28s)
=== RUN   TestAccNetworkWatcherFlowLog_tags
--- PASS: TestAccNetworkWatcherFlowLog_tags (317.00s)
PASS
ok      github.com/terraform-providers/terraform-provider-azurerm/azurerm/internal/services/network     2926.746s

[magodo]

@katbyte Would you please take a look at this PR? Some customers are blocked by this.

[katbyte]

is this working correctly? all the tests are failing in teamcity now, worse then before

[katbyte]

Thanks @magodo - LGTM 👍

[ghost]

This has been released in version 2.63.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.63.0"
}
# ... other configuration ...

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