Multiple changes to Dockerfile and startup script

- Allow installation source to be provided via env variable FABRIC8_SRC
- Default to using latest binary from sonatype maven repository
- Generate admin password during build
- Use /usr/share/fabric8 (FABRIC8_HOME) as fabric8 user's home
- Disable login for fabric8 user
- Use fabric8 user to run fabric8
- Startup script is now made available as /usr/bin/fabric8-start
- Modify startup script to handle ENABLE_SSHD and SHOW_PASSWORD
  environment variables
- Do not allow EmptyPasswords or sudo ssh access
- Sanitize user add