build-iso: switch between release and development key depending on type

Switch between the development and release keys when building the ISO,
depending on the build type selected.  This prevents the warning about
untrusted packages when building development versions, requiring user
input at this stage.  This also removes the need to replace the gpg
key used before switching build types.

Bug #250 http://bugzilla.vyos.net/show_bug.cgi?id=250
Merge branch 'gpg-keys' into helium