support file inclusion in env and service attributes

[prfalken]

fixes #52 fixes #60

inclusion should be in the form of an "include" key containing a list of includes :

include:
  - env:prod-dc1:some.config
  - env::some.config.from.same.environment
  - some.global.config

when omitting the env field like in env::some.config the current environment is used

• env:prod-dc1:some.config leads to /env/prod-dc1/common-attributes/some/config.yml
• env::some.config.from.same.environment leads to common-attributes/some/same/environment.yml of the same environment
• some.global.config leads to /common-attributes/some/global/config.yml