qa/tasks: reduce duplicated code

qa/tasks: reduce duplicated code

All `exec`-style function in teuthology appear to have a transformation
block that expands names like `all-roles` and `all-hosts`. With the new
cephadm.exec task that block appeared twice in cephadm.py. This change
removes the duplication by creating an _expand_roles function that
can be called from the command executing functions.

Signed-off-by: John Mulligan <jmulligan@redhat.com>