lttng-libust: enable Python agent when Python is available

Add the --enable-python-agent configure option to build the LTTng-UST
Python agent when Python 2 or Python 3 is also part of the BR image. The
LTTng-UST Python agent is a Python package which adds LTTng tracing to
Python's standard logging when imported.

Disable the Python agent otherwise.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>