Pulling the Environment from the Azure CLI config

[tombuildsstuff]

Fixes #662.

Tests pass:

$ go test -v
=== RUN   TestAzureFindValidAccessTokenForTenant_InvalidDate
--- PASS: TestAzureFindValidAccessTokenForTenant_InvalidDate (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_Expired
2018/02/15 08:51:40 [DEBUG] Token "7cabcf30-8dca-43f9-91e6-fd56dfb8632f" has expired
--- PASS: TestAzureFindValidAccessTokenForTenant_Expired (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_ExpiringIn
--- PASS: TestAzureFindValidAccessTokenForTenant_ExpiringIn (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_InvalidManagementDomain
2018/02/15 08:51:40 [DEBUG] Resource "https://portal.azure.com/" isn't a management domain
--- PASS: TestAzureFindValidAccessTokenForTenant_InvalidManagementDomain (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_DifferentTenant
2018/02/15 08:51:40 [DEBUG] Resource "https://management.core.windows.net/" isn't for the correct Tenant
--- PASS: TestAzureFindValidAccessTokenForTenant_DifferentTenant (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_ValidFromCloudShell
--- PASS: TestAzureFindValidAccessTokenForTenant_ValidFromCloudShell (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_ValidFromAzureCLI
--- PASS: TestAzureFindValidAccessTokenForTenant_ValidFromAzureCLI (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_NoTokens
--- PASS: TestAzureFindValidAccessTokenForTenant_NoTokens (0.00s)
=== RUN   TestAzureCLIProfileFindDefaultSubscription
--- PASS: TestAzureCLIProfileFindDefaultSubscription (0.00s)
=== RUN   TestAzureCLIProfileFindSubscription
--- PASS: TestAzureCLIProfileFindSubscription (0.00s)
=== RUN   TestAzurePopulateSubscriptionFromCLIProfile_Missing
--- PASS: TestAzurePopulateSubscriptionFromCLIProfile_Missing (0.00s)
=== RUN   TestAzurePopulateSubscriptionFromCLIProfile_NoDefault
--- PASS: TestAzurePopulateSubscriptionFromCLIProfile_NoDefault (0.00s)
=== RUN   TestAzurePopulateSubscriptionFromCLIProfile_Default
--- PASS: TestAzurePopulateSubscriptionFromCLIProfile_Default (0.00s)
=== RUN   TestAzurePopulateTenantFromCLIProfile_Empty
--- PASS: TestAzurePopulateTenantFromCLIProfile_Empty (0.00s)
=== RUN   TestAzurePopulateTenantFromCLIProfile_MissingSubscription
--- PASS: TestAzurePopulateTenantFromCLIProfile_MissingSubscription (0.00s)
=== RUN   TestAzurePopulateTenantFromCLIProfile_PopulateTenantId
--- PASS: TestAzurePopulateTenantFromCLIProfile_PopulateTenantId (0.00s)
=== RUN   TestAzurePopulateTenantFromCLIProfile_Complete
--- PASS: TestAzurePopulateTenantFromCLIProfile_Complete (0.00s)
=== RUN   TestAzurePopulateFromAccessToken_Missing
--- PASS: TestAzurePopulateFromAccessToken_Missing (0.00s)
=== RUN   TestAzurePopulateFromAccessToken_Exists
--- PASS: TestAzurePopulateFromAccessToken_Exists (0.00s)
=== RUN   TestAzureEnvironmentNames
--- PASS: TestAzureEnvironmentNames (0.00s)
=== RUN   TestAzureValidateBearerAuth
--- PASS: TestAzureValidateBearerAuth (0.00s)
=== RUN   TestAzureValidateServicePrincipal
--- PASS: TestAzureValidateServicePrincipal (0.00s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm/helpers/authentication	0.018s

[mbfrahry]

LGTM!

[ghost]

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