[HTX] Mark HTX plugin as optional

HTX plugin has limited use case in testing OpenPower systems and we may not
need to run this plugin by default.

$ ./servicereport -l

servicereport 2.2.1

The following plugins are applicable:

   Name                Tags                Description

   daemon              M                   Daemon availability checks
   fadump              M                   FADump configuration check
   htx                 O                   HTX configuration check
   package             M                   Package availability check

Tag Info:
M: Mandatory plugin (runs by default)
O: Optional plugin (use -o option to enable)

Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>

Author: sourabhjains

servicereportpkg/validate/plugins/htx.py

@@ -22,6 +22,7 @@ def __init__(self):
         Plugin.__init__(self)
         self.name = HTX.__name__
         self.description = HTX.__doc__
+        self.optional = True
         self.service_name = "htxd"
         self.installation_path = "/var/log/htx_install_path"