Support nested TOSCA functions in get_secret

[loicalbertin]

As: TOSCA Component dev
I want to: use the get_secret TOSCA function with nested TOSCA functions
So that: I can build complex stuff like { get_secret: [ concat: ["/secrets", "/myapp", "/javahome"], java_opt1=1, java_opt2=2]}

AC1: A limitation in Alien4Cloud prevent to use something else than a scalar as parameter of the get_secret function. Apart from that this feature is already functional in Yorc. So we propose to introduce an alias to the get_secret function called get_vault_secret that will not be handled and rejected by Alien.