Enable certificate rotation in security demo

[trilokgm]

In the initial version of security-demo, certificate rotation was not possible due to keystore dependency in ghostunnel. The keystore is generated one-time during deployment and during rotation of certificate and keys, the updated keystore was not being created in the scripts.

Ghostunnel supports two formats of keystore, pkcs12 and single file containing certificate and key.
To generate a single file with certificate and key, spiffe-helper must be updated to generate the single file format given the same names of certificate and key in helper.conf.