[proxy] More flexible container name -> hostname derivation

Provides to new two new flags: `--hostname-match <regexp>`
and `--hostname-replacement <replacement>`. When launching a container, its name
matched against regular expression `<regexp>`. Then, based on that match,
`<replacement>` will be used to generate a hostname, which will ultimately be
handed over to weaveDNS for registration.

The substitution is implemented with
https://golang.org/pkg/regexp/#Regexp.ReplaceAllString . Thus, references to
regexp substitution groups should be prepended with a dollar sign.

For instance, if we provide `--hostname-match '^aws-[0-9]+-(.*)$'` and
`--hostname-replacement 'my-app-$1'`, running a container named
`aws-12798186823-foo` through WeaveProxy will lead to WeaveDNS registering
`my-app-foo`.

Closes #1018

proxy/create_container_interceptor.go

@@ -58,7 +58,9 @@ func (i *createContainerInterceptor) InterceptRequest(r *http.Request) error {
 		if err := i.setWeaveWaitEntrypoint(container.Config); err != nil {
 			return err
 		}
-		if err := i.setWeaveDNS(&container, r.URL.Query().Get("name")); err != nil {
+		containerName := r.URL.Query().Get("name")
+		hostname := i.proxy.hostnameMatchRegexp.ReplaceAllString(containerName, i.proxy.HostnameReplacement)
+		if err := i.setWeaveDNS(&container, hostname); err != nil {
 			return err
 		}
 	}