add macros for default jdk provider/version; 2.021

add macros for default jdk provider/version; 2.021

benefits:
- utilizes %use_jdk hence requires only *-base-* subpackages avoiding
  conflicts in %{_bindir}
- ensures packages are built with minimal ClassDataVersion so they can
  be run on broader set of JVMs
- easy to modify without modifying spec files

usage:

  %use_default_jdk

  Summary: ...
  ...
  %buildrequires_jdk
  ...
  %build
  export JAVA_HOME="%{java_home}"

defaults to openjdk8. if package requires specific minimal version of
java pass version as argument ie %{use_default_jdk 11}