oo-admin-yum-validator: Fix string in guess_ose

Put quotation marks around the string literal 'ose' in the guess_role
method to avoid a NameError exception and a backtrace.

This commit fixes bug 1112883.
Merge pull request #389 from Miciah/bug-1112883-oo-admin-yum-validator-fix-ose-string-literal-in-guess_role

oo-admin-yum-validator: Fix string in guess_ose