Add optional hostname argument to weave expose

[awh]

Fix #375:

vagrant@ubuntu-14:~/weave$ ./weave expose 10.1.1.1/24 -h myservice.weave.local
vagrant@ubuntu-14:~/weave$ ./weave status
weave router git-59ae50eb4ef9
Encryption off
Our name is 7a:c5:c4:15:be:33(ubuntu-14)
Sniffing traffic on &{10 65535 ethwe 16:3d:0b:8a:66:e4 up|broadcast|multicast}
MACs:
7a:c5:c4:15:be:33 -> 7a:c5:c4:15:be:33(ubuntu-14) (2015-04-16 09:23:30.45427506 +0000 UTC)
6a:0a:da:89:30:de -> 7a:c5:c4:15:be:33(ubuntu-14) (2015-04-16 09:22:13.804277587 +0000 UTC)
16:3d:0b:8a:66:e4 -> 7a:c5:c4:15:be:33(ubuntu-14) (2015-04-16 09:22:03.786283726 +0000 UTC)
32:dd:63:79:54:df -> 7a:c5:c4:15:be:33(ubuntu-14) (2015-04-16 09:22:04.374157935 +0000 UTC)
Peers:
Peer 7a:c5:c4:15:be:33(ubuntu-14) (v0) (UID 5547860031843093274)
Routes:
unicast:
7a:c5:c4:15:be:33 -> 00:00:00:00:00:00
broadcast:
7a:c5:c4:15:be:33 -> []
Reconnects:


weave DNS git-59ae50eb4ef9
Local domain weave.local.
Listen address :53
mDNS interface &{14 65535 ethwe 6a:0a:da:89:30:de up|broadcast|multicast}
Fallback DNS config &{[66.28.0.45 8.8.8.8] [] 53 1 5 2}
Zone database:
weave:expose 10.1.1.1 myservice.weave.local.

There are however a number of caveats as compared to the behaviour of -h as used in conjunction with run:

• weave hide cannot remove the DNS entries in the same way that weave detach does as there is no container in which to persist the metadata
• We have to use the synthetic container ID of weave:expose when we interact with the DNS REST API (@inercia can you foresee any problems with this?)

[inercia]

@awh I think it should be ok to have synthetic names like that... in fact, I will introduce a remote container ID in my next PR, so weave:expose should not be problematic either...

[rade]

We should make hide work properly for this.

[awh]

D'oh - the REST API doesn't require the hostname so fix is trivial...