covert docker (commit|import|tag) to use REPOSITORY[:TAG]

 (and remove support for REPOSITORY TAG)


partial fix for #2294, #2149, #2387 

mostly, I wonder if the server should also refuse to set a REPOSITORY name that contains a : - thus reserving it as a separator for host, port, path and tag.
secondly, I've not written any tests, and on my system, no tests fail :( - have to get to that when i poke the other issues

Closes #2450

tell me if you want me to make this change backwards compatible - I'm not so sure its worth it in the long term, but it may break people's scripts :/