image selection rewrite

All command now use select_image in repoman to select which image should be
select for use

search is now added to repoman and not by command

best option was dropped for all command

changelog command take now more than one image
diff command is no more limited to 2 args. Can be one with globbing
list -a is dropped, use *:* instead (this can be used in all command)

is command allow smart image selection like this:

- ftp/* => list all image in repo ftp
- ftp/*:4 => list all image in version 4 in repo ftp
- *:4 => list all images at version 4 in search path
- */*:4 => list all images at version in all repo
- debian => list all images nammed debian in search path
- debian* => list all images starting by debian in search path