yum-validator: use sub-man to set repo options

subscription-manager now supports repo-overrides, which allow setting
options on RHSM repos that take precedence over what's specified in
`/etc/yum.repos.d/redhat.repo`. This commit updates the repo config
modifying routine `CheckSources.set_save_repo_attr` so that it calls out
to `/usr/sbin/subscription-manager` with appropriate options to set the
desired configuration setting in the repo overrides.

This also adds `reconcile_rhsm_config.py`, which provides o-a-y-v with a
way to report or fix settings mismatched between the local `redhat.repo`
file and repo-override settings. This is enabled with the
`--reconcile-overrides|-e` option to `oo-admin-yum-validator`

Enterprise bug: 1162296
https://bugzilla.redhat.com/show_bug.cgi?id=1162296
Updated README, man page and cleaned up some comments & doctext
Merge pull request #493 from dobbymoodge/repo_override_fix

yum-validator: use subscription-manager to set repo options
<oo-install> always set conf_node_external_eth_dev for origin
Bug 1163338 - Improve command detection and error handling

https://bugzilla.redhat.com/show_bug.cgi?id=1163338

- explicity search /sbin and /usr/sbin when looking for utilities
- improve error handling around confirm_access