Allow environment variable LTTNG_HOME to override HOME

Patch functionality - If LTTNG_HOME environment variable exists, it is
used instead of HOME.  Reason for patch - We are trying to deploy LTTng
on a system where $HOME is on a filesystem mounted read-only, but cannot
afford to run lttng as a different user and cannot move the home
directories of users to writeable locations.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>