Update snf-manage commands arguments to be more consistent

These commits update a lot of snf-manage commands:
* 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`
* Update docs to comply with the above changes
* Print default value of `--wait` option in help messages