[ebpf] Check for bpftool presence before running plugin

The plugin was running even when the bpftool was
not present, throwing an exception when it tried to parse
a json output:

INFO: [plugin:ebpf] Could not parse bpftool prog list as
JSON: Expecting value: line 1 column 1 (char 0)

It now checks if the program is present before running
any command at all.

Related: RH SUPDEV-151

Signed-off-by: Jose Castillo <jcastillo@redhat.com>

sos/report/plugins/ebpf.py

@@ -15,6 +15,7 @@ class Ebpf(Plugin, IndependentPlugin):
     short_desc = 'eBPF tool'
     plugin_name = 'ebpf'
     profiles = ('system', 'kernel', 'network')
+    commands = ('bpftool',)
 
     option_list = [
         PluginOpt("namespaces", default=None, val_type=int,