Sanitize tag values so that DNS and tools are not confused by invalid characters

My projects tends to have Name tags that are not compatible with DNS naming scheme, including dots and forward slashes. While it may work, we are at the mercy of the tools and intermediate DNS servers.
After this change Name=ha_proxy/0 becomes ha_proxy-0 and A#$%bb becomes a-bb, for example. Underscore `_` is preserved as it de-facto works universally and used by many people.
Internationalized domain names are out of scope of this change.