Increase flexibility of dockerctl shell

new internal function get_container_id can
look up container ID from many identifiers.

dockerctl shell can now accept the following
identifiers for a container:
  short name (cobbler)
  long name (fuel-core-5.1-cobbler)
  short id (e506c54d80d1)
  long id (64 chars)

Change-Id: Ia54a2a905a060848d773a9c81bbe4df1f8991d82
Partial-Bug: #1324227
Merge "Increase flexibility of dockerctl shell"