Adding unique constraint on the service name

[kumarashit]

What this PR does / why we need it: If service is created with an existing name then it should not be allowed. To solve this, we have put a unique constraint on the Service name.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #84

Special notes for your reviewer: Tested from UI that when service is created with same name then it throws 400 error and the response is " Instance name should be unique"

Release note:

[codecov-io]

Codecov Report

Merging #92 into development will decrease coverage by 0.77%.
The diff coverage is 39.13%.

@@               Coverage Diff               @@
##           development      #92      +/-   ##
===============================================
- Coverage         59.6%   58.83%   -0.78%     
===============================================
  Files               12       12              
  Lines              854      872      +18     
  Branches            89       91       +2     
===============================================
+ Hits               509      513       +4     
- Misses             304      318      +14     
  Partials            41       41

Impacted Files	Coverage Δ
orchestration/db/models.py	98.66% <100%> (+0.01%)	⬆️
orchestration/api/instances.py	52.36% <25%> (-1.18%)	⬇️
orchestration/db/api.py	68.6% <38.46%> (-2.29%)	⬇️