Change memory limit units from bytes to megabytes or to gigabytes

[ubuntolog]

Right now they are specified in bytes, which is not very convenient to modify

[emanueldima]

Maybe also change the names, add suffixes (MB and Sec)?

[ubuntolog]

Good idea. Maybe we should keep some names, since they are taken from the docker documentation and it makes them easier to understand (e.g. CpuShares, CpuPeriod)