Default configuration values are inconsistently applied

[fspmarshall]

As part of investigating #3213 it was discovered that the default CachePolicy would be different based on whether or not a config file exists (even if said config file is empty).

The config.FileConfig seems to be responsible for setting a number of default configuration values, but the defaults are never propagated if no concrete file exists.

The fix itself is relatively easy (always propagate defaults), but doing so may alter the behavior of existing teleport instances which don't use config files. I have yet to determine what (if any) configuration values other than cache policy are affected.