Get new auth token for ramdisk if old will expire soon

PXE driver prepares auth token for ramdisk, if this token near
expiration deploy will be fail. This patch modify PXE driver,
get new token from Keystone for this case. New function
token_expires_soon() added to keystone module.

Closes-Bug: #1377168
Change-Id: I215e2ec679b510152e2d140234594f7596e602af
Merge "Get new auth token for ramdisk if old will expire soon"