(maint) Update file mapping to reference yum_repo_path

Previously the file mapping here used /opt/repository/yum hardcoded, when that
path is available in the @build object as yum_repo_path. This commit updates
the section to use the variable.
(maint) Remove duplicate logic from ship_rpms task

The ship_rpms task previously did the same actions twice, once for el and once
for fedora, using the same 5 lines of code. This commit moves those 5 lines
into a loop over el and fedora so that the code is reused.
(maint) The file mapping needs a trailing slash after dist so that we don't remove the trailing slash from the original pathing.
Achievement unlocked: nitpicking over extra space
Merge pull request #224 from haus/maint/master/ship_fixups

Simple improvements to the ship task