Fix teuthology installations on physical Centos machines.
Yum installs of packages specify a pacakge number.  Initial
install of yum source changed to not fail if already done.
Added yum cleans where necessary.

Fixes: #4768
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge branch 'wip-teuth4768a-wusui'

Conflicts:
	teuthology/task/install.py