Replace usage of dsh with scap.ssh

Use the new scap.ssh module instead of DSH in scap.tasks.scap. This
allows displaying a readable progress message during the execution of
a command on the cluster hosts. Host, exit status and command output
will be logged for failures as they occur.

Bug: 59922
Change-Id: Ia3a99755ae5ef413cc61845ddc5939ff68c10cf2