Add troubleshoot option to PXE template

Ported from nova: I169a5e3b7a9d80e18e389ae4645d2c5bbdae2a6f

Diskimage-builder recently added an optional troubleshoot parameter
which defaults to 0 (false). See I2cba8a9674075ba7e420027d40ef8cbe756cf07e

This commit adds the 'troubleshoot' option to the PXE template file
so it will be easier for people to find should they need to enable
troubleshooting mode.

Change-Id: I90d2389b5afa68407e2e68923a3486c7fd9f574e
Merge "Add troubleshoot option to PXE template"