Cognito Identity identity_id format should be in format `REGION:GUID`
Hi

Running tests using moto revealed issue with the _identity id_ gotten from `cognitoidentity`. In reality aws _identity_ids_ should be `REGION:GUID`.[0] However moto creates an _identity id_ in format `"{0}:{1}".format(region, get_random_hex(length=19))`. [1] This causes issues when being strict with what the format is expected to be.

[0] https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/cognito-identity.html#CognitoIdentity.Client.get_id
[1] https://github.com/spulec/moto/blob/master/moto/cognitoidentity/utils.py
diff --git a/moto/cognitoidentity/utils.py b/moto/cognitoidentity/utils.py
index 6143d5121..54016ad17 100644
--- a/moto/cognitoidentity/utils.py
+++ b/moto/cognitoidentity/utils.py
@@ -1,5 +1,5 @@
-from moto.core.utils import get_random_hex
+from uuid import uuid4
 
 
 def get_random_identity_id(region):
-    return "{0}:{1}".format(region, get_random_hex(length=19))
+    return "{0}:{1}".format(region, uuid4())
diff --git a/tests/test_cognitoidentity/test_cognitoidentity.py b/tests/test_cognitoidentity/test_cognitoidentity.py
index 8eae183c6..0ec7acfb0 100644
--- a/tests/test_cognitoidentity/test_cognitoidentity.py
+++ b/tests/test_cognitoidentity/test_cognitoidentity.py
@@ -7,6 +7,7 @@ from nose.tools import assert_raises
 from moto import mock_cognitoidentity
 from moto.cognitoidentity.utils import get_random_identity_id
 from moto.core import ACCOUNT_ID
+from uuid import UUID
 
 
 @mock_cognitoidentity
@@ -83,8 +84,10 @@ def test_describe_identity_pool_with_invalid_id_raises_error():
 
 # testing a helper function
 def test_get_random_identity_id():
-    assert len(get_random_identity_id("us-west-2")) > 0
-    assert len(get_random_identity_id("us-west-2").split(":")[1]) == 19
+    identity_id = get_random_identity_id("us-west-2")
+    region, id = identity_id.split(":")
+    region.should.equal("us-west-2")
+    UUID(id, version=4)  # Will throw an error if it's not a valid UUID
 
 
 @mock_cognitoidentity
@@ -96,7 +99,6 @@ def test_get_id():
         IdentityPoolId="us-west-2:12345",
         Logins={"someurl": "12345"},
     )
-    print(result)
     assert (
         result.get("IdentityId", "").startswith("us-west-2")
         or result.get("ResponseMetadata").get("HTTPStatusCode") == 200
diff --git a/tests/test_cognitoidentity/test_server.py b/tests/test_cognitoidentity/test_server.py
index 903dae290..8c4229f06 100644
--- a/tests/test_cognitoidentity/test_server.py
+++ b/tests/test_cognitoidentity/test_server.py
@@ -48,6 +48,5 @@ def test_get_id():
         },
     )
 
-    print(res.data)
     json_data = json.loads(res.data.decode("utf-8"))
     assert ":" in json_data["IdentityId"]
['tests/test_cognitoidentity/test_cognitoidentity.py', 'moto/cognitoidentity/utils.py', 'tests/test_cognitoidentity/test_server.py']