Remove hard coding of /etc/avalon path in code

[manojgop]

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[danintel]

I see /etc/avalon hard-coded here:

./shared_kv_storage/setup.py:conf_dir = "/etc/avalon"
./shared_kv_storage/kv_storage/remote_lmdb/lmdb_listener.py:    return '/etc/avalon'

Directory /etc/avalon is a reasonable choice of a default config directory. Certainly better than directory$TCF_HOME/config, especially once Avalon becomes packaging and we stop running Avalon under a mixed source/object/configuration directory ($TCF_HOME). But there must be a way to override the location. For example,

• $AVALON_CONFIG with default /etc/avalon
• $AVALON_HOME with default /opt/avalon
• $AVALON_VAR with default /var/lib/avalon

This is based on the traditional UNIX/Linux practice of separating configuration, executible/read-only files, and writable data files in separate directories (and therefore possibly separate file systems).