Added CentOS install functionality.

install_packages, remove_packages, and remove_sources are now the installation
fuctions used by teuthology.  Debian references have been removed outside of
tasks/install.py.  CentOS functionality parallel to Debian has been added to
tasks/install.py.

System types are used as keys rather than an index.  Changes were also made
to make -a option work for Centos (use service to restart rsyslog,
and make sure all directories exist when copying log files).