Add check for pipeline configuration reload

[martialblog]

Adds new subcheck for pipelines to validate the last successful reload time.

I thought using a subcheck was a cleaner solution than to mix it with the inflight event checks.

Fixes #49

Code can be optimized in the future, there is some duplication. But better than wrongs abstraction.

Examples:

$ check_logstash pipeline reload
OK - Configuration successfully reloaded
 \_[OK] Configuration successfully reloaded for pipeline Foobar for on 2021-01-01T02:07:14Z

$ check_logstash pipeline reload --pipeline Example
CRITICAL - Configuration reload failed
 \_[CRITICAL] Configuration reload for pipeline Example failed on 2021-01-01T02:07:14Z

[martialblog]

@widhalmt FYI