Store and pull tenant creds from Key Vault.

The tenant ID should be hashed and used as the key for the JSON blob of
relevant creds for any given tenant. Azure CSP interface methods that
need to source creds should call the internal `_source_creds` method,
either with a `tenant_id` or no parameters. That method will source the
creds. If a tenant ID is provided, it will source them from the Key
Vault. If not provided, it will return the default creds for the app
registration in the home tenant.

tests/utils/test_hash.py

@@ -0,0 +1,16 @@
+import random
+import re
+import string
+
+from atst.utils import sha256_hex
+
+
+def test_sha256_hex():
+    sample = "".join(
+        random.choices(string.ascii_uppercase + string.digits, k=random.randrange(200))
+    )
+    hashed = sha256_hex(sample)
+    assert re.match("^[a-zA-Z0-9]+$", hashed)
+    assert len(hashed) == 64
+    hashed_again = sha256_hex(sample)
+    assert hashed == hashed_again