Description:
------------
Some tests have code to find number of CPUs. But MTR have already
found that number, but has not exported it for use in tests or in
include files. Export an environment variable to hold number of
CPUs and use it in tests.

Fix:
----
- Introduced a new enironment variable $NUMBER_OF_CPUS containing
  the number of CPUs value.

- Rewritten mysql-test/include/resource_group_init.inc file to use
  the new environment variable $NUMBER_OF_CPUS.

Change-Id: Ife84d89227ffefd452f3baa5882fc9fa958ef05f