Scap/Scap3 deployment groups

This feature was introduced in differential revision D16

In addition to the dsh_targets config variable, scap looks for multiple [anything]_dsh_targets config variables. This enables canary_dsh_targets.

All additional deployment groups will be executed before the primary deployment group (defined by the dsh_targets variable).

Additionally, checks now can be scoped to a specific deployment group using:

check_name:
    stage: promote
    group: dsh-group-name
    command: touch /tmp/hi-there

The group name is optional in a check. If not group name is specified, check runs for all deploy groups.