SSH: Add host key pem generator

CRaSH is used on multiple servers (we, specifically, use it as standalone),
so it is needed to have a different host key per installation.
For this, we find it somewhat a requirement to generate the host key per
installation, either at boot or when someone first connects through SSH
(Apache's SSHD examples are using SimpleGeneratorHostKeyProvider).

This patch adds the option to automatically generate a host key, by demand.
A new option (ssh.keygen) in crash.properties was added.

I've made sure this patch is backward compatible. It's also OK to first use
it with "ssh.keygen=true" and then disable it. The key is generated with
PEMGeneratorHostKeyProvider so it should work with URLKeyPairProvider, once
disabled.

I have also tried to make the change set as small as possible.
Comment out ssh.keygen property
Add ssh.keygen property to documentation
CRASH-161 : https://jira.exoplatform.org/browse/CRASH-161
[maven-release-plugin] prepare release 1.3.0-beta12
[maven-release-plugin] prepare for next development iteration
dashboard: Remove GC spaces list print

When using the dashboard command, a list of gc pools is printed to screen
before the main table layout is shown.

This is caused because of `(jvm.pools | { name -> .. })()`. The closure, in
this case, returns the name of the pool in question. This causes it to be
printed to screen while the UIBuilder.table() is generated.

Explicitly placing "null" at the end of the closure makes Groovy consider this
as the return value, thus nothing gets printed to screen before the dashboard
is displayed.
CRASH-162 : dashboard remove GC spaces list print