Restrict length of repository names

The registry restricts the length of repository names
to <30 characters. This causes the Engine to match
the registry's restriction, limiting repository names to
29 characters. If a repository has a namespace (i.e. user), this
only applies to the name right of the /.

For instance the following would be considered 29 characters:

 * 12345678901234567890123456789
 * ewindisch/12345678901234567890123456789

Signed-off-by: Eric Windisch <eric@windisch.us>