[yang]: AAA login pattern (#9805)

Signed-off-by: Gang Lv ganglv@microsoft.com

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#### Why I did it
end2end test is blocked by Yang model for AAA login pattern.

#### How I did it
Add pattern to AAA yang models.

#### How to verify it
Run UT for sonc-yang-models.

#### Which release branch to backport (provide reason below if selected)

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- Note we only backport fixes to a release branch, *not* features!
- Please also provide a reason for the backporting below.
- e.g.
- [x] 202006
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- [ ] 201811
- [ ] 201911
- [ ] 202006
- [ ] 202012
- [ ] 202106

#### Description for the changelog
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Write a short (one line) summary that describes the changes in this
pull request for inclusion in the changelog:
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Fix #9713 

#### A picture of a cute animal (not mandatory but encouraged)

src/sonic-yang-models/tests/yang_model_tests/tests/aaa.json

@@ -6,6 +6,10 @@
         "desc": "Configure a wrong type in AAA table.",
         "eStrKey": "InvalidValue"
     },
+    "AAA_TEST_WRONG_LOGIN": {
+        "desc": "Configure a wrong type in AAA table.",
+        "eStr": ["Invalid login choice"]
+    },
     "AAA_TEST_WRONG_FAILTHROUGH": {
         "desc": "Configure a wrong failthrough in AAA table.",
         "eStrKey": "Pattern",

src/sonic-yang-models/tests/yang_model_tests/tests_config/aaa.json

@@ -24,6 +24,16 @@
         }
     },
 
+    "AAA_TEST_WRONG_LOGIN": {
+        "sonic-system-aaa:sonic-system-aaa": {
+            "sonic-system-aaa:AAA": {
+                "AAA_LIST": [{
+                        "login": "locallll"
+                }]
+            }
+        }
+    },
+
     "AAA_TEST_WRONG_FAILTHROUGH": {
         "sonic-system-aaa:sonic-system-aaa": {
             "sonic-system-aaa:AAA": {

src/sonic-yang-models/yang-models/sonic-system-aaa.yang

@@ -30,8 +30,12 @@ module sonic-system-aaa {
                 }
 
                 leaf login {
-                    type string;
-                    description "AAA authentication/authorization/accounting methods - local/tacacs+/disable";
+                    type string {
+                        pattern '((tacacs\+|local|radius|default),)*(tacacs\+|local|radius|default)' {
+                            error-message "Invalid login choice";
+                        }
+                    }
+                    description "AAA authentication/authorization/accounting methods - radius/tacacs+/local/default";
                     default "local";
                 }