Fix the bug of docker import command when import a image with a tag use format 'URL|- [REPOSITORY[:TAG]]'

In latest version 1.1.2,when use command 'docker import' to import a image with a tag use format 'URL|- 
[REPOSITORY[:TAG]]',it will return a error message shows that the repository name is invalid,for example:

[root@localhost images]# tar -c . | docker import - test:v1
2014/08/21 08:12:33 Invalid repository name (test:v1), only [a-z0-9-_.] are allowed

In previous version,it will parse the Arg[1] which value is 'REPOSITORY:TAG' ,get the REPOSITORY and TAG,then check if REPOSITORY can be resolved,but in the 1.1.2 version when import a image with a tag use format 'URL|- [REPOSITORY[:TAG]]',it will check the variable 'repository' which value is Arg[1] directly,but in fact the value of variable 'repository' is 'REPOSITORY:TAG' which contains a colon and colon is not in character set [a-z0-9-_.],so the error occurred.
I add codes to call parsers.ParseRepositoryTag function to parse variable 'repository' and return the REPOSITORY which store in variable 'repo',then check the variable 'repo'.

Fixes #7714