Skip tests that would fail if mercurial or p4python aren't installed.
Skip tests that would fail if mercurial or p4python aren't installed.

When the hostingsvcs tests got added, they weren't added with sufficient checks
to avoid a bunch of failures if the underlying packages weren't installed. After
this change there are still a bunch of failures if pysvn isn't present, but
that's another bag of fish.

1tush/reviewboard,beol/reviewboard,reviewboard/reviewboard,davidt/reviewboard,beol/reviewboard,custode/reviewboard,1tush/reviewboard,KnowNo/reviewboard,brennie/reviewboard,1tush/reviewboard,beol/reviewboard,brennie/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,chipx86/reviewboard,KnowNo/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,1tush/reviewboard,1tush/reviewboard,reviewboard/reviewboard,custode/reviewboard,custode/reviewboard,KnowNo/reviewboard,brennie/reviewboard,chipx86/reviewboard,brennie/reviewboard,1tush/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,sgallagher/reviewboard,davidt/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,davidt/reviewboard,reviewboard/reviewboard,beol/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,1tush/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,custode/reviewboard,KnowNo/reviewboard,chipx86/reviewboard