Updated constant definitions, they are imported from tuf.developer_tool
This makes it easier to propagate design changes through the two
different tools.

A tuf.constants module would be a good addition to this.
release_signable was renamed to project_signable
release_signable didn't make too much sense in such context and was
renamed to a more accurate and descriptive variable name.
file open is now done with the "with" statement
The with statement is a more "native" way to do the same thing, it is
done that way not.