Add config entry for SRU regression emails

Also rename the key to SRUREGRESSIONEMAIL_ENABLE, since
SRUREGRESSION_ENABLE sounds like we're turning off the regressions
themselves. If only. 😜

britney.conf

@@ -104,3 +104,4 @@ ADT_SHARED_RESULTS_CACHE =
 
 # email uploaders for stuck uploads
 EMAIL_ENABLE     = yes
+SRUREGRESSIONEMAIL_ENABLE = no

britney.py

@@ -544,7 +544,7 @@ class Britney(object):
             self.policies.append(EmailPolicy(self.options,
                                              self.suite_info,
                                              dry_run=add_email_policy == 'dry-run'))
-        add_sruregression_policy = getattr(self.options, 'sruregression_enable', 'no')
+        add_sruregression_policy = getattr(self.options, 'sruregressionemail_enable', 'no')
         if add_sruregression_policy in ('yes', 'dry-run'):
             self.policies.append(SRUADTRegressionPolicy(self.options,
                                                         self.suite_info,