Permit to build OVS with only Python3 installed

Permit to build OVS with only Python3 installed

This commit renames HAVE_PYTHON to HAVE_PYTHON2 and PYTHON to PYTHON2
and adds HAVE_PYTHON and PYTHON with a different semantics:
- If PYTHON environment variable is set, use it as PYTHON
- If a python2 interpreter is available, PYTHON became the python2 interpreter
- If a python3 interpreter is available, PYTHON became the python3 interpreter

PYTHON is only used to run the python scripts needed by the build system

NOTE:
Since currently most of the utilities and bugtool doesn't support Python3,
they're installed only if python2 is available. This will be fixed in later
commits.

Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>