ipalib: remove hard dependency on ipapython

Hard-code the path to /bin/false in SubprocessError doc string.

Remove ipaplatform dependency from ipalib's setup.py and add it as optional
installer dependency to ipalib's and ipaclient's setup.py.

https://fedorahosted.org/freeipa/ticket/6474

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

ipalib/errors.py

@@ -108,7 +108,6 @@ import six
 
 from ipalib.text import ngettext as ungettext
 from ipalib import messages
-from ipaplatform.paths import paths  # pylint: disable=unused-import
 
 
 class PrivateError(Exception):
@@ -151,7 +150,7 @@ class SubprocessError(PrivateError):
     The exit code of the sub-process is available via the ``returncode``
     instance attribute.  For example:
 
-    >>> e = SubprocessError(returncode=1, argv=(paths.BIN_FALSE,))
+    >>> e = SubprocessError(returncode=1, argv=('/bin/false',))
     >>> e.returncode
     1
     >>> e.argv  # argv is also available