cyclades: Update argument names of snf-manage cmds

* Update various snf-manage commands to use "--user" instead of
"--owner/--userid/--user-id"
* Update server-create and server-import to use --flavor instead of
--flavor-id
* Update server-create and server-import to use --image instead of
--image-id
* Update subnet-create to use --network instead of --network-id
docs: Update docs to comply with previous commit

* Update docs and Changelog to reflect changes introduced in the
previous commit.