Fix entrypoint.sh creating one folder named {plugins,embedded-db}

Summary:
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.

Test Plan:

$ export OPENFIRE_DATA_DIR=/tmp/openfire
$ mkdir -p "${OPENFIRE_DATA_DIR}"/{plugins,embedded-db}
$ ls $OPENFIRE_DATA_DIR
embedded-db  plugins

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2372