Do not log at debug log level when HA_debug is unset

There might be situations (e.g. bundles) where the HA_debug variable
is unset. It makes little sense to enable debug logging when the HA_debug env
variable is unset.
So let's skip debug logs when HA_debug is set to 0 or is unset.

Tested inside a bundle and observed that previously seen 'ocf_log debug'
calls are now correctly suppressed (w/ HA_debug being unset inside the
container)

Signed-off-by: Michele Baldessari <michele@acksyn.org>