Retry POSTing logs if KeyError occurs on iterating over env variables

[rplevka]

addresses #39
Add a simple retry loop to catch intermittent race condition occurring when py.test unsets PYTEST_CURRENT_TEST variable during urllib's iterating over env. variables

[krasoffski]

9 - Looks like "magic" number. Let's create module level variable.

Need Update

[krasoffski]

Need to remove magic number.

[DzmitryHumianiuk]

@rplevka up

[rplevka]

@krasoffski @DzmitryHumianiuk - comments addressed.
I'm wondering though, whether is should capture all exceptions instead since the variable is called: POST_LOGBATCH_RETRY_COUNT. but in reality this will currently only retry the post_batch on a KeyError.
Or, suggest a new name for the variable, however POST_LOGBATCH_RETRY_COUNT_ON_KEY_ERROR sounds ridiculous ;D

[DzmitryHumianiuk]

@krasoffski good to merge?