add --find-if-exists to create-space and create-org

Add a `--find-if-exists` option to `cf create-space` and `cf create-org`, which will not blow up if a user tries to create a space in an org with a name that already exists, or tries to create an org with a name that already exists.

The motivation is to allow people to write more idempotent scripts that may need to shell out and invoke `cf`.

-- @Amit-PivotalLabs + @jfoley