Allow colon in backend names in overrides

[kepi]

Citing official documenationt at https://docs.haproxy.org/2.6/configuration.html#4

All proxy names must be formed from upper and lower case letters, digits, '-' (dash), '_' (underscore) , '.' (dot) and ':' (colon). ACL names are case-sensitive, which means that "www" and "WWW" are two different proxies.

I didn't do extensive testing, we have only couple cases where we are using overrides. So please approach this as simple "works for us" and better double check.

[phibos]

Thanks for providing the fix.