Fixing alert filter using alertname

[pavangudiwada]

Fixes

Traceback (most recent call last):                                                                         
alertmanager                                                                                               
    issues = source.fetch_issues()                                                                         
             ^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                     
line 96, in fetch_issues                                                                                   
    alertname_filter = re.compile(self.alertname)                                                          
                                  ^^^^^^^^^^^^^^                                                           
AttributeError: 'AlertManagerSource' object has no attribute 'alertname'. Did you mean:                    
'username'?     

[aantn]

LGTM