fix(service): add support for using default values in template parameters

[Panaetius]

SwissDataScienceCenter/renku-project-template@56aa0a2 added a template parameter to our templates, breaking RP tests. This PR adds proper support for using default values for template parameters.

[coveralls]

Pull Request Test Coverage Report for Build 5466058021

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 128 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.2%) to 85.508%

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Files with Coverage Reduction	New Missed Lines	%
renku/core/errors.py	1	83.52%
renku/core/migration/m_0003__1_jsonld.py	1	87.68%
renku/core/util/tabulate.py	1	96.3%
renku/core/workflow/activity.py	1	93.61%
renku/ui/cli/dataset.py	1	89.77%
renku/ui/cli/init.py	1	96.9%
renku/ui/service/controllers/templates_create_project.py	1	95.18%
renku/core/dataset/dataset.py	6	76.3%
renku/core/dataset/tag.py	9	74.29%
renku/core/util/communication.py	9	91.85%

Totals
Change from base Build 5462986423:	-0.2%
Covered Lines:	25720
Relevant Lines:	30079

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💛 - Coveralls