Support uploading multiple certificates [install-backend]

To enable multiple certificates support in certificate manager,
and resolve following issues :

 openbmc/phosphor-certificate-manager#3,
 openbmc/bmcweb#84

the Install method should return a string with dbus path to just
created certificate. It will allow to recognize a new certificate
and return proper values in redfish response to certificate POST
method.

This change depends on interface change available under this review:
 https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-dbus-interfaces/+/25632

Signed-off-by: Zbigniew Kurzynski <zbigniew.kurzynski@intel.com>
Depends-On: I58bfb8a86f56923e7f7aca0d10ed7838537d7c14
Change-Id: Ic8ab545c931d89a022acdabd5a755e3bf39e5a79
diff --git a/certs_manager.hpp b/certs_manager.hpp
index da47d35..91c5e43 100644
--- a/certs_manager.hpp
+++ b/certs_manager.hpp
@@ -62,7 +62,7 @@
      *
      *  @param[in] filePath - Certificate key file path.
      */
-    void install(const std::string filePath) override;
+    std::string install(const std::string filePath) override;
 
     /** @brief Delete the certificate (and possibly revert
      *         to a self-signed certificate).