exclude all netty subprojects
exclude all netty subprojects

Commit a937891 adds a dependency on netty-all:4.1.5.Final to attempt to
fix the memory leak issue in 4.1.3.Final, but does not exclude the
actual 4.1.3 dependencies that the pubsub client pulls in.

Just to avoid any possible surprises about what versions of the
classfiles end up in the final shaded jar for helios-agent or
helios-master, be extra sure about removing the 4.1.3 netty dependencies
that google-cloud-pubsub drags in.

Output of `mvn dependency:tree -Dverbose` before this commit:

```
[INFO] +- com.google.cloud:google-cloud-pubsub:jar:0.5.1:compile
...
[INFO] |  +- io.grpc:grpc-netty:jar:1.0.1:compile
[INFO] |  |  +- io.netty:netty-codec-http2:jar:4.1.3.Final:compile
[INFO] |  |  |  +- io.netty:netty-codec-http:jar:4.1.3.Final:compile
[INFO] |  |  |  |  \- io.netty:netty-codec:jar:4.1.3.Final:compile
[INFO] |  |  |  |     \- (io.netty:netty-transport:jar:4.1.3.Final:compile - omitted for duplicate)
[INFO] |  |  |  \- io.netty:netty-handler:jar:4.1.3.Final:compile
[INFO] |  |  |     +- io.netty:netty-buffer:jar:4.1.3.Final:compile
[INFO] |  |  |     |  \- io.netty:netty-common:jar:4.1.3.Final:compile
[INFO] |  |  |     +- io.netty:netty-transport:jar:4.1.3.Final:compile
[INFO] |  |  |     |  +- (io.netty:netty-buffer:jar:4.1.3.Final:compile - omitted for duplicate)
[INFO] |  |  |     |  \- io.netty:netty-resolver:jar:4.1.3.Final:compile
[INFO] |  |  |     |     \- (io.netty:netty-common:jar:4.1.3.Final:compile - omitted for duplicate)
[INFO] |  |  |     \- (io.netty:netty-codec:jar:4.1.3.Final:compile - omitted for duplicate)
...
```

and after:

```
[INFO] +- com.google.cloud:google-cloud-pubsub:jar:0.5.1:compile
...
[INFO] |  +- io.grpc:grpc-netty:jar:1.0.1:compile
[INFO] |  |  \- io.grpc:grpc-core:jar:1.0.1:compile
( no more netty jars)
...
```

spotify/helios,spotify/helios,mavenraven/helios,mattnworb/helios,mavenraven/helios,mattnworb/helios,mattnworb/helios,mavenraven/helios,spotify/helios