Bug 2082: Store Snapcraft Crdentials as Secret

Changes the credendtials used for the snapstore from being in a
file in jenkins' home directory to being a secret stored in
the Jenkins server itself.

Fixes bug 2082

Change-Id: Icb78ac46c83bdab1176c9316482f713f7bd89e4b
Signed-off-by: beierlm <mark.beierl@canonical.com>