Azure SP docs (#514)

* Adding Azure SP related doc

* Fixing title

* Fixing nit

Author: savitharaghunathan

docs/config/azure_plugin.md

@@ -0,0 +1,117 @@
+<hr style="height:1px;border:none;color:#333;">
+<h1 align="center">Authentication Methods for Azure Plugin </h1>
+
+### Using Service Principal Credentials
+1. To use Service Principal as authentication mechanism, create a credential file using the following command,
+```
+cat << EOF  > ./credentials-velero
+AZURE_SUBSCRIPTION_ID=${AZURE_SUBSCRIPTION_ID}
+AZURE_TENANT_ID=${AZURE_TENANT_ID}
+AZURE_CLIENT_ID=${AZURE_CLIENT_ID}
+AZURE_CLIENT_SECRET=${AZURE_CLIENT_SECRET}
+AZURE_RESOURCE_GROUP=${AZURE_RESOURCE_GROUP}
+AZURE_STORAGE_ACCOUNT_ACCESS_KEY=${AZURE_STORAGE_ACCOUNT_ACCESS_KEY}
+AZURE_CLOUD_NAME=AzurePublicCloud
+EOF
+```
+
+<b>Note:</b> 
+- Make sure to include `AZURE_STORAGE_ACCOUNT_ACCESS_KEY` in the credentails file, as it is required by the Azure registry pod to perform registry related operations.
+
+2. Once you have the credentials file, create the secret using the following command,
+
+```
+oc create secret generic cloud-credentials-azure --namespace openshift-adp --from-file cloud=./credentials-velero
+```
+
+3. Create a DataProtectionApplication (DPA) CR with the appropriate values. For example, the DPA CR would look like,
+```
+apiVersion: oadp.openshift.io/v1alpha1
+kind: DataProtectionApplication
+metadata:
+  name: dpa-sample
+spec:
+  backupLocations:
+    - velero:
+        config:
+          resourceGroup: <resource_group_name>
+          storageAccount: <storage_account>
+          subscriptionId: <subscription_id>
+        credential:
+          key: cloud
+          name: cloud-credentials-azure
+        default: true
+        objectStorage:
+          bucket: <bucket_name>
+          prefix: velero
+        provider: azure
+  configuration:
+    restic:
+      enable: true
+    velero:
+      defaultPlugins:
+        - openshift
+        - azure
+  snapshotLocations:
+    - velero:
+        config:
+          resourceGroup: <resource_group_name>
+          subscriptionId: <subscription_id>
+        provider: azure
+```
+
+### Using Storage Account Access Key Credentials
+
+1. create a credential file using the following command,
+```
+cat << EOF  > ./credentials-velero
+AZURE_STORAGE_ACCOUNT_ACCESS_KEY=${AZURE_STORAGE_ACCOUNT_ACCESS_KEY}
+AZURE_CLOUD_NAME=AzurePublicCloud
+EOF
+```
+
+2. Once you have the credentials file, create the secret using the following command,
+
+```
+oc create secret generic cloud-credentials-azure --namespace openshift-adp --from-file cloud=./credentials-velero
+```
+
+3. Create a DataProtectionApplication (DPA) CR with the appropriate values. For example, the DPA CR would look like,
+```
+apiVersion: oadp.openshift.io/v1alpha1
+kind: DataProtectionApplication
+metadata:
+  name: dpa-sample
+spec:
+  backupLocations:
+    - velero:
+        config:
+          resourceGroup: <resource_group_name>
+          storageAccount: <storage_account>
+          subscriptionId: <subscription_id>
+          storageAccountKeyEnvVar: AZURE_STORAGE_ACCOUNT_ACCESS_KEY
+        credential:
+          key: cloud
+          name: cloud-credentials-azure
+        default: true
+        objectStorage:
+          bucket: <bucket_name>
+          prefix: velero
+        provider: azure
+  configuration:
+    restic:
+      enable: true
+    velero:
+      defaultPlugins:
+        - openshift
+        - azure
+  snapshotLocations:
+    - velero:
+        config:
+          resourceGroup: <resource_group_name>
+          subscriptionId: <subscription_id>
+        provider: azure
+```
+
+<b>Note:</b> 
+If you would like to take backups to the specified VolumeSnapshotLocation, make sure to include Service Principal credentials.