Add format options to 'nova coverage-report'.

This adds --html and --xml options to 'nova coverage-report'
to enable selection of coverage report format.
If specifying none of these, text-formatted report will be created.

This also adds --combine option to 'nova coverage-start' to make sure
that 'nova coverage-report --html' works fine.

Fixes bug 1114766

Change-Id: I9fee26bd5c45cac35f425ac7abbced4e2f3ff4df