Fix(?) for DockerTemplate volumes param

This commit fix a problem with the "volumes" parameter in
DockerTemplate. Appears to be an issue parsing the "volumeString"
constructor param.

With docker-plugin-0.6 (Docker 1.0.0), seeing 500 on POST to
/containers/{id}/start:

Jun 16, 2014 1:53:59 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 11 * Sending client request on thread
Computer.threadPoolForRemoting [#3]
11 > POST
http://localhost:4243/containers/065824b714bb1865d4a293867b66d29aff164b93ff877a5b108199b0c0388014/start
11 > Content-Type: application/json
{"Binds":[""],"PortBindings":{"22/tcp":[{"HostIp":"0.0.0.0"}]},"Privileged":false,"Dns":[""]}

Jun 16, 2014 1:53:59 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 12 * Client response received on thread
Computer.threadPoolForRemoting [#3]
12 < 500
12 < Content-Length: 87
12 < Content-Type: text/plain; charset=utf-8
12 < Date: Mon, 16 Jun 2014 01:53:59 GMT
Could not create local directory '' for bind mount: mkdir : no such file
or directory!

After this fix, getting 204 as expected:

Jun 16, 2014 2:41:29 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 3 * Sending client request on thread
Computer.threadPoolForRemoting [#3]
3 > POST
http://localhost:4243/containers/9158b1c4d44cebc8c86deb04952cd66d852b27428aa28878c92a5b03804b1f60/start
3 > Content-Type: application/json
{"PortBindings":{"22/tcp":[{"HostIp":"0.0.0.0"}]},"Privileged":false,"Dns":[""]}

Jun 16, 2014 2:41:30 AM org.glassfish.jersey.filter.LoggingFilter log
INFO: 4 * Client response received on thread
Computer.threadPoolForRemoting [#3]
4 < 204
4 < Content-Length: 0
4 < Content-Type: text/plain; charset=utf-8
4 < Date: Mon, 16 Jun 2014 02:41:30 GMT
Merge branch 'fix/volume_param' of https://github.com/hyleung/docker-plugin into hyleung-fix/volume_param

Conflicts:
	pom.xml
[maven-release-plugin] prepare release docker-plugin-0.6.1
[maven-release-plugin] prepare for next development iteration