Add a fallback check for the 2.x backup configuration

[lazypower]

When moving forward etcd runs a configuration transaction and renames the original 2.x series config to etcd.conf.2x. I feel it makes sense to look for the 2.x configuration and use it as a fallback mechanism before defaulting to error should the user attempt to snap revert etcd

If accepted closes #1

[markshuttle]

OK, thanks, will merge.