[VOL-3386] Add support for secure gRPC in openolt-agent

The init script of the openolt service may start with '--enable-tls <TLS_OPTION>' argument for the gRPC server.
Default is insecure with no '--enable-tls' argument.
The TLS capability depends upon the certificates stored at the keystore/ directory: 1. root.crt (CA public key), 2. server.crt (public key), 3.server.key (private key).
Four unit tests are added for the secure gRPC server which work with the keystore-test/ directory.
The certificates stored at the keystore-test/ directory are self-signed certificates, valid until Apr 11 23:16:58 2031 GMT.

Change-Id: I4d18a98a0193f501f922360c79f54b0fcedf14a5
diff --git a/agent/test/keystore-test/root.crt b/agent/test/keystore-test/root.crt
new file mode 100644
index 0000000..2ae357b
--- /dev/null
+++ b/agent/test/keystore-test/root.crt
@@ -0,0 +1,24 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----