Add 'cache_dir' option to api.env
    
    `api.env` now has a `cache_dir` option, which defaults to
    `os.path.join(USER_CACHE_PATH, 'ipa')`. Schema cache, server info, and
    KRA key cache use `api.env.cache_dir` as base directory. The option
    allows application to set a custom cache directory.
    
    Related: https://pagure.io/freeipa/issue/9438
    Related: https://bugzilla.redhat.com/show_bug.cgi?id=1513934
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>