Automatically call the host secrets plugin

[carolynvs]

Right now when Porter resolves values for a Parameter/Credential set, we call the secrets plugin for everything, even those where the source is not secret, like env, path, etc. Those other sources are all handled by the host secrets plugin and we shouldn't require every custom secrets plugin to handle figuring out when to call back to the host secrets plugin.

When resolving a ps/cs, Porter should check the source, and if it is a source that is associated with the host secrets plugin: command, value, env, path, then Porter should resolve it directly from the host secrets plugin. The secrets plugin should only be called for other sources. Right now the only other source is secret, but if in the future we add more source types, then we should also allow custom plugins to handle that if they choose.