MINOR: logs: add a new per-proxy "log-tag" directive

MINOR: logs: add a new per-proxy "log-tag" directive

This is equivalent to what was done in commit 48936af ("[MINOR] log:
ability to override the syslog tag") but this time instead of doing
this globally, it does it per proxy. The purpose is to be able to use
a separate log tag for various proxies (eg: make it easier to route
log messages depending on the customer).