insert-ethers: streamlined command invocation

now it is easier to make self.commands.command('some.command', args)
inside insert-ethers
pylib: added rocks check services to verify running services

rocks check services check that all the services required for
installing nodes are running, otherwise returns error.

insert-ethers invoke this command at every startup
pylib: do not run exit in commands, raise an exception instead

when a command fails it uses self.abort which before this patch
was running exit(1), this make difficult reusing commands
functionalities in other rocks modules

With this patch commands always raise an execption instead of
exiting, then it up to the invoker to decide what to do
pylib: always use Rocks exceptions for rocks error

before this patch some functions were using ValueError and other
python default exception which was creating confusion.

Now rocks related exception are always mapped to Rocks exception
in rocks.util, so there is no confusion