add global Kerberos propagation

[VMois]

closes #167

How to test:

• follow https://github.com/reanahub/reana-workflow-controller/pull/448 instructions

• update reana.yaml

diff --git a/reana.yaml b/reana.yaml
index 50d1bd8..2b9126b 100644
--- a/reana.yaml
+++ b/reana.yaml
@@ -10,14 +10,13 @@ inputs:
     sleeptime: 0
 workflow:
   type: serial
+  resources:
+    kerberos: true
   specification:
     steps:
-      - environment: 'python:2.7-slim'
+      - environment: 'reanahub/krb5'
         commands:
-          - python "${helloworld}"
-              --inputfile "${inputfile}"
-              --outputfile "${outputfile}"
-              --sleeptime ${sleeptime}
+          - klist 2>&1; exit 0
 outputs:
   files:
    - results/greetings.txt

[codecov-commenter]

Codecov Report

Merging #169 (790f8b3) into master (eb2ecc1) will increase coverage by 1.00%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   27.46%   28.47%   +1.00%     
==========================================
  Files           5        5              
  Lines         142      144       +2     
==========================================
+ Hits           39       41       +2     
  Misses        103      103              

Impacted Files	Coverage Δ
reana_workflow_engine_serial/tasks.py	0.00% <0.00%> (ø)
reana_workflow_engine_serial/config.py	100.00% <100.00%> (ø)