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
Corrects 2nd argument type for logging

Fixes a regression introduced in commit
c73afa9fd1df14bff9186c1e73ac8f3593ef81db.

This completely breaks Horizon in last
DevStack version.

Change-Id: Ib754c6ea325534399a34ff76a290475ac652fea9
Fixes: bug #1122958
Merge "Corrects 2nd argument type for logging"