Allow updating to a specific Autolab version

[damianhxy]

Currently, make update pulls the latest Autolab (and Tango) versions. Also, it doesn't pull the latest changes from the docker repo.

However, this might be undesirable as the master branches could be unstable.

Possible approach

• Create autolab/docker releases in tandem with autolab/Autolab releases
• make update to support optional specification of autolab/docker release tag. If specified, it should checkout the docker repo at that tag and pull the corresponding submodules (git submodule update)

[GuillaumeDIDIER]

I second this suggestion, running on master shouldn't be the default.