give install_modules options for loading requirements externally

Use case:  Our team would like to use install_modules.sh
  in other places beyond openstack-infra/config, we would
  also like to provide some control over what install_modules.sh
  installs beyond what is given as defaults.

  * Added a copyright notice so we can use it in other projects
  * if a modules.env file is found in the same directory
    as install_modules.sh then source the modules.env
  * an export called MODULE_ENV_FILE can be included to
    augment the name of the file from modules.env
    to an alternative file name like modules.test.env.
    This gives users of the script an option to provide
    different modules on different test environments.
  * as export to change the path from the default
    directory to an alternate directory can be
    specified with the export MODULE_ENV_PATH var.
    The default directory is the same location as
    the install_modules.sh script.

Change-Id: I0c3f9bcb3e91236ceca492426ee6105242a2c8eb
Merge "give install_modules options for loading requirements externally"