Saltstack howto create a Concatenation CA?
It feels like what you have done here is implement a discount/bastardized CA (Certificate Authority). I fear that in doing so, you might have inadvertently created a security threat for yourself.
In a traditional PKI deployment, your client would generate a private key and a Certificate Signing Request (CSR) that is then submitted to your Certificate Authority for signing. Your CA would then sign your key after validating the applicant by some means and issue a Certificate to be installed on the client.
What you may have just done by concatenating every single certificate of each client into a CA is grant each and every client the ability to sign certificates for other machines. Any machine in your Gluster cluster then may have the ability to issue a certificate for an arbitrary key which then would be trusted by your Gluster cluster which could allow for an attacker to use a compromised system or key to allow an arbitrary endpoint to join your cluster.
Instead, you might want to follow this guide on setting up a CA for use by your SaltStack. In fact, it is worth noting that your salt master is itself a CA. You could simply copy the minion's certificate and key for use by gluster, though that has the downside of a compromised cert/key being able to be used to impersonate a minion.
But generally speaking, you probably don't want to grant all of your minions the power to be a CA.