Remove unnecessary ssh-credentials defaults

ODL used to use a variable called {ssh-credentials} as a form of
redirection in order to have a simple human readable word to represent a
unique credentials id in Jenkins. Newer versions of Jenkins actually
allows us to override the credentials-id UUID format and pass a human
readable filename.

Considering that FD.io has been using a human readable
jenkins-gerrit-credentials credentials-id remove the unnecessary
{ssh-credentials} id defaults from global-defaults and just access the
credential directly.

Change-Id: I7b59357dcacb2195646719709b3b623429deca91
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>