Fix some small problems in RepoParser

Fix some small problems in RepoParser

RepoParser.parse() is a bit of hard for understanding. try to make
it more cleaner and readable.

 - remove useless self.archs
 - split local path out first, otherwise urlgrab will complain
    "<url> malformed" for local path
 - get_repos_by_arch() should return local+standard repos at least
 - check self.buildconf each time before getting repo's build.conf
 - self.repos is used only in one place in self.parse(), it can
    be a local var and passed by argument
 - build meta of one repo should not be used for another repo, so
    self.buildmeta can also be a local var in for loop
 - add a function is_standard_repo() to check repomd.xml file
 - add a function fetch() to fetch a remote file into self.cachedir

Change-Id: I1d5d8f9347a5426a323aa04e7edf8c5e75c1c65a