Deployment will fail if we have no public IPs on instances

If user wants to deploy Hello World Java Cluster, he should set checkbox 'assign floating IP' for Tomcat and PostgreSQL applications instances, because if user will not do this, deployment will fail.
Looks like we need to change logic in Java Cluster workflow and convert $.tomcat.public_ip to string with the separate operator and use internal IP address if user will no assign floating IPs on the instances.

Steps To Reproduce:
1. Create new environment 'test'.
2. Add Tomcat and PostgreSQL applications without floating IPs to environment 'test'.
3. Add Java Cluster Hello World application to environment 'test' and select created Tomcat and PostgreSQL applications.
4. Start to deploy this environment.

Observed Result:
Deployment will fail with the following error in log:

[exceptions.TypeError]: unsupported operand type(s) for +: 'NoneType' and 'str'

In Murano Engine we can see the following traceback:

2014-09-16 03:39:22.011 6562 ERROR murano.common.engine [-]
  exceptions.TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
  Traceback (most recent call last):
    File "/tmp/murano-packages-cache/d97fcac3-5f23-4a7e-91bf-96373e813a71/io.murano/Classes/Environment.yaml", line 67:13 in method deploy of class io.murano.Environment
        $.applications.pselect($.deploy())
    File "/tmp/murano-packages-cache/d97fcac3-5f23-4a7e-91bf-96373e813a71/io.murano.apps.java.HelloWorldCluster/Classes/HelloWorldCluster.yaml", line 55:22 in method deploy of class io.murano.apps.java.HelloWorldCluster
        $.tomcat.public_ip + ':8080/' + $.dir
    File "/home/murano/murano/murano/murano/dsl/helpers.py", line 66 in method evaluate
        return execute_instruction(value, func, context)
    File "/home/murano/murano/murano/murano/dsl/helpers.py", line 52 in method execute_instruction
        result = action()
    File "/home/murano/murano/murano/murano/dsl/helpers.py", line 62 in method <lambda>
        func = lambda: evaluate(value.evaluate(context), context, 1)
    File "/home/murano/murano/murano/murano/dsl/yaql_expression.py", line 73 in method evaluate
        return self._parsed_expression.evaluate(context=context)
    File "/home/murano/murano/murano/.tox/venv/local/lib/python2.7/site-packages/yaql/expressions.py", line 35 in method evaluate
        return f()
    File "/home/murano/murano/murano/.tox/venv/local/lib/python2.7/site-packages/yaql/expressions.py", line 100 in method __call__
        return self._try_invoke(fs, args_to_pass, context)
    File "/home/murano/murano/murano/.tox/venv/local/lib/python2.7/site-packages/yaql/expressions.py", line 112 in method _try_invoke
        return func(*args_to_pass)
    File "/home/murano/murano/murano/.tox/venv/local/lib/python2.7/site-packages/yaql/functions/builtin.py", line 118 in method plus
        return a() + b()
    File "/home/murano/murano/murano/.tox/venv/local/lib/python2.7/site-packages/yaql/expressions.py", line 100 in method __call__
        return self._try_invoke(fs, args_to_pass, context)
    File "/home/murano/murano/murano/.tox/venv/local/lib/python2.7/site-packages/yaql/expressions.py", line 112 in method _try_invoke
        return func(*args_to_pass)
    File "/home/murano/murano/murano/.tox/venv/local/lib/python2.7/site-packages/yaql/functions/builtin.py", line 118 in method plus
        return a() + b()