created yumvalidator lib dir, moved modules there
fix module references for yumvalidator
remove global repo_cache declaration
Move repo data out of repo_db.py

Created `yum-validator/etc` directory to hold config data

Added `RepoDBError` for signaling problems instantiating a `RepoDB`

Updated `repo_db.py` to check `/etc/yum-validator/repos.ini` and
`./etc/repos.ini` for default repository data, and raise `RepoDBError`
if file can't be found or loaded.
Catch `RepoDBError` and bail

Add code to catch `RepoDBError` during `OpenShiftYumValidator`
instantiation and bail, since no repo data means no worky