Add a health action to sites controller to check g5k-api availability

Fixes: bug #13404
3 jobs for features/#13404 in 17 minutes and 37 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Lint
passed #1364712
grid5000-docker
rubocop

00:01:31

 
  Test
passed #1364713
grid5000-docker
test-for-buster

00:08:10

80.48%
 
  Deb
passed #1364714
grid5000-docker
deb-for-buster

00:07:55