Allow customization of commonEventHeadres before request is sent

Allow customization of commonEventHeadres before request is sent

Details:
- Python handler is allowed to take --data argument with fields to be overriden.
- In python handler the original dictionary from config.json is merged with custom sourceName
 and reportingEntityName if provided.
- Java source code generates default sourceName and reportingEntityName as part of default data in
the form of "default-XXX", where XXX is YEAR, MONTH and DAY as integers.
- If such default data is received in the request body, don't override them with default values.
- BUG FIXED: the previous solution used abstract path that could be "/" that resulted in empty strings
for sourceName and reportingEntityName.

Issue-ID: INT-1800
Signed-off-by: Eli Halych <illia.halych@t-mobile.pl>
Change-Id: Ic88c5606a44abac64f842f28621332f3f86b83c6