Merge lp://staging/~ltrager/maas-images/rename_centos into lp://staging/maas-images

Rename centos products to include 'daily'

 * Renames the CentOS images to include 'daily' in the product name
   This follows the convention of the ubuntu daily images.
 * Fixes a bug when the SHA256 sums don't match meph2-util would crash
   because it was trying to read the wrong variable
 * Adds the sign command to sign the stream with your gpg key

This patch does the following

* Renames the CentOS images to include 'daily' in the name
* Fixes a bug when the SHA256 sums don't match meph2-util would crash because it was trying to read the wrong variable
* Adds the sign command to sign the stream with your gpg key

We can rename the existing daily stream with the following

STREAM_PATH="/path/to/daily/streams/v1"
rm ${STREAM_PATH}/*.sjson ${STREAM_PATH}/*.json.gpg
mv "${STREAM_PATH}/com.ubuntu.maas:centos-bases-download.json" \
    "${STREAM_PATH}/com.ubuntu.maas:daily:centos-bases-download.json"
sed -i \
    -e 's/com.canonical.maas:centos-bases/com.ubuntu.maas.daily:centos-bases/g' \
    -e 's/"label": "release"/"label": "daily"/g' \
    ${STREAM_PATH}/*.json
meph2-util sign /path/to/daily

I've confirmed that after using the above script to rename the centos images the import command does not regenerate the centos images and MAAS does not redownload them.

I was also able to confirm that the following command successfully promotes a centos image into a stream which currently has none
meph2-util promote --no-sign --label=release /path/to/daily /path/to/releases 20141129_01 release=centos70 product_name=com.ubuntu.maas.daily:centos-bases:7.0:amd64