[elk] Add option to fetch from selected branches

[inishchith]

Fixes: #642

@jgbarah @valeriocos Please do have a look, let me know if any changes required.

Thanks!

TODO:

• Add tests (if needed)
• Update docstrings

Signed-off-by: inishchith inishchith@gmail.com

[coveralls]

Pull Request Test Coverage Report for Build 1512

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 216 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 68.218%

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Files with Coverage Reduction	New Missed Lines	%
/home/travis/build/chaoss/grimoirelab-elk/grimoire_elk/raw/elastic.py	16	43.31%
/home/travis/build/chaoss/grimoirelab-elk/grimoire_elk/elk.py	200	11.71%

Totals
Change from base Build 1510:	-0.1%
Covered Lines:	5628
Relevant Lines:	8250

---

💛 - Coveralls

[valeriocos]

Thank you @inishchith for the PR, do you plan to add tests? I can have a look at the code and help you with this.

[inishchith]

@valeriocos I'll add the tests and ping you back.

Thanks!

[inishchith]

@valeriocos Sorry, I'm not sure if tests could be added as we do not have tests covering the ocean_backend.feed() method. Please do correct me in case I missed something.

[valeriocos]

No worries, @inishchith I'll have a look and see if we can add some tests

[valeriocos]

It isn't straightforward to add tests since the code should be refactored.

[valeriocos]

LGTM! Thanks @inishchith

[valeriocos]

@sduenas I tested the PR and it works fine.

The change is similar to the one to include the filter-classified param (210e877), so we shouldn't have surprises.

Tests cannot be easily added because the code should be refactored. If you agree we can give it a try otherwise the PR can be accepted as it is.

[sduenas]

LGTM. Let's merge it.