mgr: avoid false alarm of MGR_MODULE_ERROR

mgr: avoid false alarm of MGR_MODULE_ERROR

mgr sends healthy report periodically, the report includes the
information whether the always-on modules are loaded or not. but the
modules are loaded with two steps:

1. load the options and command exposed by modules. the options and
   commands are registered using static methods of the subclasss of
   MgrModule.
2. create an instance of the subclass of MgrModule. this is performed
   in background by a Finisher thread. upon finishing of the construction
   of the instance, ActivePyModules::start_one() adds the module which
   successfully creates the class to `modules`.

but there is chance that when mgr sends healthy report, the always-on
module is still creating its instance of MgrModule subclass, or that
task is still pending in the finisher thread. in that case, mgr would
add a false error message like
```
4 mgr modules have failed (MGR_MODULE_ERROR)
```
in the healthy report

in this change, the number of modules in pending state is tracked,
and mgr will not take the missing always-on modules into account unless
the number of pending modules is 0.

Signed-off-by: Kefu Chai <kchai@redhat.com>