(CPR-83) Extend rsync_from to include extra flags

In a previous commit, the rsync_to method was updated to include
arbitrary extra flags to facilitate overriding the default extra_flags
of --ignore-existing. This commit applies the same change to the
rsync_from method, except there is no default set of extra flags.
Merge pull request #347 from haus/ticket/master/cpr_83_add_extra_flags_to_rsync_from

(CPR-83) Extend rsync_from to include extra flags