Add option required=(True|False) to get_bin_path and update modules

Added required as optional argument to get_bin_path(). It defaults to
false.  Updated following modules to use required=True when calling
get_bin_path():  apt_repository, easy_install, group, pip,
supervisorctl, and user.
Also removed _find_supervisorctl() from supervisorctl module and updated
_is_running() to not need it.

lib/ansible/module_common.py

@@ -202,9 +202,12 @@ class AnsibleModule(object):
         log_args = re.sub(r'login_password=.+ (.*)', r"login_password=NOT_LOGGING_PASSWORD \1", log_args)
         syslog.syslog(syslog.LOG_NOTICE, 'Invoked with %s' % log_args)
 
-    def get_bin_path(self, arg, opt_dirs=[]):
+    def get_bin_path(self, arg, required=False, opt_dirs=[]):
         '''
         find system executable in PATH.
+        Optional arguments:
+           - required:  if executable is not found and required is true, fail_json
+           - opt_dirs:  optional list of directories to search in addition to PATH
         if found return full path; otherwise return None
         '''
         sbin_paths = ['/sbin', '/usr/sbin', '/usr/local/sbin']
@@ -223,6 +226,8 @@ class AnsibleModule(object):
             if os.path.exists(path) and os.access(path, os.X_OK):
                 bin_path = path
                 break
+        if required and bin_path is None:
+            self.fail_json(msg='Failed to find required executable %s' % arg)
         return bin_path
 
     def boolean(self, arg):