add submodules definition

.gitmodules

+[submodule "bc-tls-patched"]
+	path = certificate-generator/bc-tls-patched
+	url = https://github.com/eID-Testbeds/bc-tls-patched.git
+[submodule "common-testbed-utilities"]
+	path = certificate-generator/common-testbed-utilities
+	url = https://github.com/eID-Testbeds/common-testbed-utilities.git