Create Docker Config for Secret Vars only if the variables were changed

[bohdan-shulha]

We can not access the value of the secret variables, but we can access their names.

Need to compare "previous" and "current" set of the variables and if these set match and no new values are provided - reuse the previous config.

We can consider dropping the configs creation for secret vars, as they are applied as env vars on the container anyway.

Looks like we might not need to create docker configs to persist this data at all, as, if a user would need to restore the previous state, secrets will be there. See encryption ticket for details: #17

[bohdan-shulha]

Not needed anymore - there are no "secret vars configs" ptah-sh/ptah-agent#51