Changing inputs as a optional parameter in job-invocation

[adarshdubey-star]

Apidoc snippet:

job_invocation[inputs]optional , nil allowed | Inputs to useValidations:Hash

Usage:

job = entities.JobInvocation().run(
            data={
                'job_template_id': 89,
                'inputs': {'command': 'ls'},
                'targeting_type': 'static_query',
                'search_query': f'name = {rhel7_contenthost.hostname}',
            }
        )

I came across this when I was trying to invoke an ansible job on a host, which raises an key error for not having an input.

[adarshdubey-star]

The issue with the CI failure is taken care of in this PR by @sambible
#841

[codecov-commenter]

Codecov Report

Base: 93.85% // Head: 93.85% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (6b9c66e) compared to base (bd34895).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #866      +/-   ##
==========================================
- Coverage   93.85%   93.85%   -0.01%     
==========================================
  Files           6        6              
  Lines        2899     2898       -1     
==========================================
- Hits         2721     2720       -1     
  Misses        178      178              

Impacted Files	Coverage Δ
nailgun/entities.py	93.31% <100.00%> (-0.01%)	⬇️

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