Multi-arch build

[imikushin]

Build for arm or amd64 by running dapper against a docker host on the target architecture.

[imikushin]

rancher/rancher#4703

[imikushin]

@ibuildthecloud Honestly, don't know what's wrong with the tests

[cjellick]

@imikushin looks like the test script is running through all the vendor dirs. can you fix?

PASS
ok      github.com/rancher/host-api/stats   11.494s
?       github.com/rancher/host-api/testutils   [no test files]
?       github.com/rancher/host-api/util    [no test files]
?       github.com/rancher/host-api/vendor/github.com/Sirupsen/logrus   [no test files]
?       github.com/rancher/host-api/vendor/github.com/dgrijalva/jwt-go  [no test files]
?       github.com/rancher/host-api/vendor/github.com/docker/docker/opts    [no test files]
?       github.com/rancher/host-api/vendor/github.com/docker/docker/pkg/archive [no test files]
?       github.com/rancher/host-api/vendor/github.com/docker/docker/pkg/fileutils   [no test fil
...

[ibuildthecloud]

@imikushin I squashed and carried this PR in #83 so I could fix up the deps