rgw: don't specify a length when convert bl -> string

rgw: don't specify a length when convert bl -> string

in rgw_admin we're assigning a string from a bufferlist by explicitly
specifying a size and using assign, in this case the whole contents of
the buffer are copied regardless of whether the null character is seen.
Just use normal assign which would only copy until the null is
encountered

Fixes: http://tracker.ceph.com/issues/20037
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>