ctdb-scripts: Update detect_init_style to use /etc/os-release

ctdb-scripts: Update detect_init_style to use /etc/os-release

/etc/os-release is quite universal.  It can be found on most Linux
distros and on FreeBSD.

Attempt to use /etc/os-release to detect Red Hat, SUSE and Debian
based distros.  If /etc/os-release exists but distro is unknown then
$ID is printed as the detected distro, which will probably result in
sub-optimal behaviour, but when tracing it will at least indicate that
a new distro needs to be handled.

The only way to handle missing /etc/os-release is to set
CTDB_INIT_STYLE - see ctdb.sysconfig(5) for details.

The event script unit tests are updated to use /etc/os-release so
the new logic is exercised.

Signed-off-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>