Template instanceCap is ignored when defined in job config

- [ 1.1.3 ] docker-plugin version you use
- [ 2.89.4 ] jenkins version you use
- [17.12.0-ce ] docker engine version you use

OK this one is a bit weird and I would submit a PR if I even remotely understood why this behavior is different. Long story short the instanceCap setting when defined in an individual build job always defaults to 1 and cannot be overridden, where as when defined globally in the main config it defaults to no cap which can be overridden.

Relevant config used in both job and main configs:
```XML

test

2
robot-framework



/home/jenkins

EXCLUSIVE

10


1.2.3.4:4567/docker/robot_framework:jenkins









false
false
false


true
PULL_ALWAYS


```