cooker.py: Fix key expansion issues in showVersions

cooker.py: Fix key expansion issues in showVersions

bitbake -s was not displaying correct version information when the
PREFERRED_VERSION string contains other variables. The actual built
versions would differ since the providers.py functions were called
with expanded keys at this point.

This patch expands keys for showVersions bringing everything into
sync correctly.

[YOCTO #1493]

(Bitbake rev: 3a0f2dda3c6de993f08ed50e9d513add9407339c)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>