[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

test/680_proxy_hostname_derivation.sh

@@ -0,0 +1,32 @@
+#! /bin/bash
+
+. ./config.sh
+
+C1=10.2.0.78
+C2=10.2.0.34
+C3=10.2.0.57
+CNAME1=qiuds71y827hdi-seeone-1io9qd9i0wd
+NAME1=seeone.weave.local
+CNAME2=124DJKSNK812-seetwo-128hbaJ881
+NAME2=seetwo.weave.local
+CNAME3=doesnotmatchpattern
+NAME3=doesnotmatchpattern.weave.local
+
+start_container() {
+  proxy docker_on $HOST1 run "$@" -dt $DNS_IMAGE /bin/sh
+}
+
+start_suite "Hostname derivation through container name substitutions"
+
+weave_on $HOST1 launch-dns 10.2.254.1/24
+weave_on $HOST1 launch-proxy --hostname-match '^[^-]+-(?P<appname>[^-]*)-[^-]+$' --hostname-replacement '$appname'
+
+start_container -e WEAVE_CIDR=$C1/24 --name=$CNAME1
+start_container -e WEAVE_CIDR=$C2/24 --name=$CNAME2
+start_container -e WEAVE_CIDR=$C3/24 --name=$CNAME3
+
+assert_dns_a_record $HOST1 $CNAME1 $NAME2 $C2
+assert_dns_a_record $HOST1 $CNAME2 $NAME3 $C3
+assert_dns_a_record $HOST1 $CNAME3 $NAME1 $C1
+
+end_suite