To allow spaces in variables like OPENFIRE_DATA_DIR, they have been enclosed
in quotes.
As the expression uses GNU coreutils mkdir {} syntax, put the {} in quotes
has broken the directory creation: instead to create /path/to/openfire/plugins
and /path/to/openfire/embedded-db, it created the exact quoted content.
That breaks new installations when directories don't exist.
This change fixes that regression. This is a follow-up for e76b3cfd9f47.