circle.yml: disable GCE builds when credentials are missing
circle.yml: disable GCE builds when credentials are missing

Another call to gce.sh was added in commit abebb0f197f3 ("Build new test VM
(to pickup latest Docker version)") but without the guard on
$SECRET_PASSWORD. Add the guard to be able to test on CircleCI without
Weaveworks' credentials.

alban/scope,weaveworks/scope,paulbellamy/scope,dilgerma/scope,paulbellamy/scope,weaveworks/scope,weaveworks/scope,kinvolk/scope,dilgerma/scope,kinvolk/scope,alban/scope,kinvolk/scope,alban/scope,alban/scope,kinvolk/scope,paulbellamy/scope,dilgerma/scope,alban/scope,paulbellamy/scope,alban/scope,dilgerma/scope,dilgerma/scope,kinvolk/scope,paulbellamy/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,paulbellamy/scope,dilgerma/scope,weaveworks/scope