This patch allow user to set boot source of composed node. Two
parameters are needed in request body,
"Target" - the boot source to be used at next boot
"Enabled" - options "Once" and "Continuous" are supported, which
indicate whether this action is only one time change
Partially-Implements blueprint node-action
Change-Id: I46342c884e3cb8f00069e7e2c10c878582a2b185
be919e10a5