BlueBrainNexusStore: fix tagging schemas

[MFSY]

• tagging schemas required a specific schema (https://bluebrain.github.io/nexus/schemas/shacl-20170720.ttl) URI to be present in the request URL
• '_' is present as schema in case of UNCONSTRAINED resource

[codecov-commenter]

Codecov Report

Merging #327 (0b20a2d) into master (a1f7242) will increase coverage by 0.14%. The diff coverage is 85.29%.

@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
+ Coverage   72.62%   72.77%   +0.14%     
==========================================
  Files          90       90              
  Lines        5761     5785      +24     
==========================================
+ Hits         4184     4210      +26     
+ Misses       1577     1575       -2     

Flag	Coverage Δ
unittests	72.77% <85.29%> (+0.14%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
kgforge/specializations/stores/bluebrain_nexus.py	27.01% <0.00%> (ø)
kgforge/specializations/stores/nexus/service.py	58.39% <81.81%> (+1.83%)	:arrow_up:
kgforge/specializations/stores/demo_store.py	38.41% <100.00%> (ø)
...sts/specializations/stores/test_bluebrain_nexus.py	94.01% <100.00%> (+0.67%)	:arrow_up: