provider/azurerm: Add support for storage container name validation

[stack72]

Fixes #6846

make testacc TEST=./builtin/providers/azurerm TESTARGS='-run=TestValidateArmStorageContainerName'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/azurerm -v -run=TestValidateArmStorageContainerName -timeout 120m
=== RUN   TestValidateArmStorageContainerName
--- PASS: TestValidateArmStorageContainerName (0.00s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/azurerm        0.012s

[jen20]

This needs running through goimports

[jen20]

Left two comments in line but other than that LGTM!

[stack72]

@jen20 made the 2 small changes you asked for - will merge when (if?) Travis goes green :)

[ghost]

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