Load flume-env.sh from flume_conf_dir environment variable / system property as opposed to bin directory

When frequently trying out different installations / upgrades of flume, more than not, the flume installation directory (including ./bin) is nuked and replaced with an updated version for testing, but the flume settings are usually stored under FLUME_CONF_DIR , independently to preserve the settings.

In the current code, the flume-env.sh(.template) is present in BINDIR to override.

We want to provide a mechanism, to source from FLUME_CONFIG_DIR/flume-env.sh though, (if present) while being backward compatible still.