Add config-download-dir flag to cell export command
Add config-download-dir flag to cell export command

`overcloud cell export` command does not have option
to provide from where to copy export data if config-download
data is exported to custom directory.

This change add the option for `overcloud cell export` command
to provide custom directory.

Change-Id: I8e2a0922662c0a4d913469fcf2b3180d31512b0f

openstack/python-tripleoclient