Support for PPC64 grub2 netboot

Bug: 1113816
Change-Id: Icb08cbfe44ad85e8c4a34a645a77e3a1bcf2361a
Fix for regression that recipe-task can not be cancelled by CLI

bug: 1124756
Change-Id: Ic1f46e75137e2c81c76f26553f2e42174ef39024
fix mistake in db downgrade notes for 0.17

We need to add back the lab_controller.user_id index in the same ALTER
TABLE statement, because the column is involved in an InnoDB foreign key
and InnoDB requires an index on it.

    ERROR 1553 (HY000): Cannot drop index 'uc_user_id': needed in a foreign key constraint

Change-Id: Idff5fce194fca70c58a6dbf22cf048635895e8e2
set netboot_method= for GRUB2

Change-Id: Ia748f219af045aa6709ceef16cdb32a21584c917
release notes for 0.17.3

Change-Id: I2495e7b5ef841b7b95b792f79eafeea281d8d4ee
Automatic commit of release 0.17.3
Merge branch 'release-0.17' into develop