Change primary template identification to be its name, not its AMI.

Many useful configurations will have multiple slave templates that use
the same AMI (but differ in other aspects). This patch changes various
parts of the code to properly identify templates by their names (in the
'description' field) instead of their AMI. As an example, the manual slave
provisioning button, before this patch, would allow the user to select a
template from a drop-down list, but then launched the slave using the
template's AMI, which could have resulted in the wrong template being
used for the launch. This patch also changes the global config page so
that the 'description' field is the first field in each template
definition, to emphasize its importance.

Addresses [JENKINS-7960] and [JENKINS-15158].