lib: Add support for temporary verbose mode
lib: Add support for temporary verbose mode

We want to display the output of shell commands that the user issues,
since they are likely to be important. Without this, we get only progress
output with something like 'chromite sh ls'. Since there is no progress
information, this means no output.

This is a dev convenience to avoid the need for specifying -v3 on every
shell command.

BUG=chromium-os:26802
TEST=tested with later commit: 'chromite sh ls' and see that the
command output is not supressed even with the default verbosity
setting.
Change-Id: Ie04651912dcda19c5e579058b9e15a95239bcc01
Reviewed-on: https://gerrit.chromium.org/gerrit/16618
Tested-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite