deploy: Change BootLoaderSpec filenames so they can be used for sorting

deploy: Change BootLoaderSpec filenames so they can be used for sorting

Currently the BLS snippets are named ostree-$ID-$VARIANT_ID-$index.conf,
but the BLS config files are actually sorted by using the version field
which is the inverse of the index.

In most places, _ostree_sysroot_read_boot_loader_configs() is used to
get the BLS files and this function already returns them sorted by the
version field. The only place where the index trailing number is used is
in the ostree-grub-generator script that lists the BLS files to populate
the grub config file.

But for some bootloaders the BLS filename is the criteria for sorting by
taking the filename as a string version. So on these bootloaders the BLS
entries will be listed in the reverse order.

To avoid that, change the BLS snippets filename to have the version field
instead of the index and also to have the version before deployment name.

Make the filenames to be of the form ostree-$version-$ID-$VARIANT_ID.conf
so the version is before the deployment name.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Closes: #1654
Approved by: cgwalters