cros deploy: add --root option for user to specify the installation root
cros deploy: add --root option for user to specify the installation root

This allows user to override the package installation root. Also, cros
deploy does not attempt to remount rootfs if the given root (--root)
is already writable.

BUG=chromium:341708
TEST=`cros deploy --root=/usr/local`

Change-Id: I4b0f24e4e9042e6f499e3d61fb873580dc5813a0
Reviewed-on: https://chromium-review.googlesource.com/185708
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

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