Make certificateKeyAlias value configurable from nuxeo.conf

When adding certificates for Nuxeo server to a certificate truststore, we need to add new certificates under the name "tomcat" so that they would work properly with Nuxeo, and normally that would not be an issue if we would have only 1 Nuxeo server or multiple truststores

but when we have multiple servers using the same truststore, keystore, then each Nuxeo server should have a different alias for their certificates.