Add support to enable variable substitution with environment variables

[wslulciuc]

This PR now allows environment variable access from config.yml. For example:

database:
  driverClass: org.postgresql.Driver
  url: jdbc:postgresql://${POSTGRESQL_HOST:localhost}:${POSTGRESQL_PORT:5432}/${POSTGRESQL_DB_NAME}
  user: ${POSTGRESQL_USER}
  password: ${POSTGRESQL_PASSWORD}

A UndefinedEnvironmentVariableException is thrown when a variable defined in config.yml cannot be interpolated. An example error message might be:

The environment variable 'POSTGRESQL_PASSWORD' is not defined; could not substitute the expression '${POSTGRESQL_PASSWORD}'.

[codecov]

Codecov Report

Merging #82 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #82      +/-   ##
============================================
+ Coverage     60.62%   60.67%   +0.04%     
  Complexity      158      158              
============================================
  Files            40       40              
  Lines           795      796       +1     
  Branches         77       77              
============================================
+ Hits            482      483       +1     
  Misses          271      271              
  Partials         42       42

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/marquez/MarquezApp.java	80.43% <100%> (+0.43%)	7 <1> (ø)	⬇️

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