Qualify TftpDiVersion with the suite.

Qualify TftpDiVersion with the suite.

This allows the version to differ e.g. between Wheezy and Jessie.

Update production-config* to set TftpDiVersion_jessie instead of just
TftpDiVersion, also add TftpDiVersion_wheezy using the version
replaced in commit f610ea162836 "Switch to Debian 8.0 (jessie) as OS
for test hosts".

In mfi-common we need to check for TftpDiVersion_$suite (_$guestsuite)
and TftpDiVersion manually since getconfig In that context will not
see any DebianSuite override in the environment.

This ensures that when a non-default suite is configured a
corresponding useful version of DI is selected.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>