Introduce new class Gitlab::DockerImageMemoryMeasurer.
DockerImageMemoryMeasurer accept 'image_reference' as input, it runs
this docker image and measure memory usage when the container is 'hot'.
The output is metrics of: memory(USS/PSS/RSS) usage per component.

Update CI config file to add new jobs to measure obmnibus start memory,
including: 1)triggered image by developer; 2)nightly image; 3)tag image.