htmlresult plugin: deal with missing requirements at module load time

This is how I believe *plugin* dependencies should be handled. If
something that a plugin requires is not present, it simply fails to
load.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/job.py

@@ -45,10 +45,10 @@ from avocado.settings import settings
 
 try:
     from avocado.core.plugins import htmlresult
+    HTML_REPORT_SUPPORT = True
 except ImportError:
     HTML_REPORT_SUPPORT = False
-else:
-    HTML_REPORT_SUPPORT = htmlresult.HTML_REPORT_CAPABLE
+
 
 _NEW_ISSUE_LINK = 'https://github.com/avocado-framework/avocado/issues/new'
 

avocado/core/plugins/htmlresult.py

@@ -20,13 +20,7 @@ import shutil
 import sys
 import time
 import subprocess
-
-try:
-    import pystache
-except ImportError:
-    HTML_REPORT_CAPABLE = False
-else:
-    HTML_REPORT_CAPABLE = True
+import pystache
 
 from avocado import runtime
 from avocado.core import exit_codes
@@ -282,9 +276,6 @@ class HTML(plugin.Plugin):
     enabled = True
 
     def configure(self, parser):
-        if HTML_REPORT_CAPABLE is False:
-            self.enabled = False
-            return
         self.parser = parser
         self.parser.runner.output.add_argument(
             '--html', type=str,