cmd/principal: addtoroots reorg and test.

(1) addtoroots shouldn't be a subcommand of the store, it has nothing to
do with the store. So move it out to the top level, similar to the
method on security.Principal

(2) add a test for it.

Change-Id: I3ba87226112fbc01f6da08e63584428452fd446a
4 files changed