Add special "_all" key to apply to all service environments

[sethvargo]

This makes it easier (and less error-prone) to set a common configuration like LOG_LEVEL on all services at once.

Release Note

Add special "_all" key to apply to all service environments. The special key `_all` will apply to all services. This is useful for common configuration like log-levels. A service-specific configuration overrides a value in `_all`. There are no default values for `_all`, so the default behavior is unchanged.