[Service Discovery] Fix API endpoint without path

[guicassolato]

What this PR does / why we need it:

It's currently not possible to set the annotation discovery.3scale.net/path to empty string or "/".

Which issue(s) this PR fixes

THREESCALE-1587

Verification steps

• Set the discovery.3scale.net/path annotation of a service to "/" or empty string
• Import the service from OpenShift into 3scale
• system-sidekiq container should raise an error:

WARN: TypeError: no implicit conversion of nil into String
WARN: /opt/system/app/models/service_discovery/cluster_service.rb:36:in `join'

Special notes for your reviewer:

[codecov]

Codecov Report

Merging #357 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
+ Coverage   92.58%   92.58%   +<.01%     
==========================================
  Files        2340     2340              
  Lines       75661    75673      +12     
==========================================
+ Hits        70048    70060      +12     
  Misses       5613     5613

Impacted Files	Coverage Δ
...est/unit/service_discovery/cluster_service_test.rb	98.3% <100%> (+0.43%)	⬆️
app/models/service_discovery/cluster_service.rb	96.96% <100%> (ø)	⬆️

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