Move ipalib.install.kinit to ipalib
    
    - kinit helpers are now in `ipalib.kinit`.
    - helpers can now use default ccache locations like many other similar
      helpers
    - helpers return the result from `run` for debugging
    - constants are now in `krb_utils`
    - helpers pass `KRB5*` and `GSS*` env vars along, so `KRB5_TRACE` works
    - document how to kinit for `ipalib.api`
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>