sh2: allow custom filters
sh2: allow custom filters

this allows overriding filters used,
leaves STRIP_ANSI as default,
adds TIMESTAMPS as optional builtin
(adds timestamps to whole plain text log,
 not via slow-to-render jenkins plugin to partial console log)

code for the timestamp filter is same as upstreams tsfilter
- https://github.com/openstack-infra/devstack-gate/blob/master/functions.sh#L91

and at the same time allows also passing custom filters e.g.:

- sh2(filters: ['STRIP_ANSI', 'rev | wc -c'], script: 'date')
- sh2(filters: ['STRIP_ANSI', 'TIMESTAMPS'], script: 'packstack')

Change-Id: I9bf233bf544830399600b3f44262aa8bd332a5a5

pycontribs/powertape,pycontribs/powertape