Removing the --format option from the REST API had the side effect of
also removing it from the 'plugin ls' ssh command.
Removing the option from the 'plugin ls' is a breaking change for some
users, so put it back.
This reverts commit 3bbd3c313f.
Change-Id: I2f4af48f16273f4875b28349e5afd707f2103bec