Use env var IPA_CONFDIR to get confdir
    
    The environment variable IPA_CONFDIR overrides the default confdir path.
    The value of the environment variable must be an absolute path to an existing
    directory. The new variable makes it much simpler to use the 'ipa'
    command and ipalib with a local configuration directory.
    
    Some scripts (e.g. servers, installers, and upgrades) set the confdir
    explicitly and do not support the env var.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>