Support environment variables as an alternative to any system properties

[jampukka]

System properties [{context_path}.]hakuna.config.path can now be configured via {CONTEXT_PATH}_HAKUNA_CONFIG_PATH or HAKUNA_CONFIG_PATH env variables.

env variables take higher precedence, but only within the same key, meaning in here the priority is (see the added unit test):

1. env variable {CONTEXT_PATH}_HAKUNA_CONFIG_PATH
2. system property {context_path}.hakuna.config.path
3. env variable HAKUNA_CONFIG_PATH
4. system property hakuna.config.path

Related issue #50

[tsallinen]

Looks good to me!

Although HAKUNAPI_HAKUNA is a bit redundant, perhaps this should be left as it is for backwards compatibility?

[jampukka]

Yes it's redundant as I didn't want to change the System.property key in 1.0.0 away from [{context_path}.]hakuna.config.path.

Alternatively, as these are already hakuna prefixed, just accept that these keys have a legacy behind them and drop the HAKUNAPI_ prefix from env variables altogether.

[jampukka]

Changed it now @tsallinen