correct various issues with glance images and configs

- update keystone endpoints/user on boot (to avoid problems caused by,
  e.g., a stale password or invalid endpoint urls)
- require GLANCE_DB_PASSWORD and GLANCE_KEYSTONE_PASSWORD in the
  environment, since we start multiple containers

Change-Id: I31214b81280ed34409f92e79003c1116d5737d2e