Credentials are lost when explicitly set

[andrewpmartinez]

Experienced

If credentials in a ziti context config are explicitly set, they are lost.

Expected

Credentials are used if set. If not set, default to checking for an identity config.

Reproduction

1. Create some credentials certCreds := &edgeapis.CertCredentials{...}
2. Set them on a ztx config: ziti.Conf{Credentials: certCreds}
3. Attempt to authenticate ztx.Authenticate()