Clamp pm_password to 255 for nova-bm nodes

Due to nova If26b648dee95abc5aa0bf80d3111265b00141b6d, pm_password
is now enforced to be 255 characters, so clamp pm_password to 255
characters before creating the node.

Change-Id: Id5c489eb7d6ef555c588207e7f5ba34a55d33000
Closes-Bug: #1322599
Merge "Clamp pm_password to 255 for nova-bm nodes"