Add a 5 minute connection timeout to ssh slave launch() function.

Some improvements to ssh slave launch logging.
Remove untranslated messages from properties files.
Add configuration option to set the ssh launch timeout in seconds.

Leaving it blank or setting it to 0 will disable timeouts.
Merge branch 'connect_timeout' of github.com:pure-marcmac/ssh-slaves-plugin

Also tidy-up of the code being merged

Conflicts:
	src/main/java/hudson/plugins/sshslaves/SSHConnector.java
	src/main/java/hudson/plugins/sshslaves/SSHLauncher.java