Added el6 install functionality for CentOS systems.

Added el6 install functionality for CentOS systems.

install_packages, remove_packages and remove_sources are now the
installation and removal functions used by teuthology.  Debian
references have been removed outside of tasks/install.py.  CentOS
functionality parallel to Debian have been added to tasks/install.py,
and el6 references have been added to nuke.py, task/ceph-fuse.y and
task/install.py.

Some files created by CentOS are removed with rm -fr.  This should
be changed once the installation/removal rpm procedure is implemented.

Signed-off-by: Warren Usui <warren.usui@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>