Bug 998955 and Bug 998925

Changed validation rules for environment variable names.

Altough the POSIX standards doesn't define the lexical convention for variable names, most shell implementations follow the Shell and Utilities volume of IEEE Std 1003.1-2001: *environment variables consist solely of letters, digits, and the '_' (underscore) from the characters defined in Portable Character Set and do not begin with a digit.*

This is also going to be implemented on the REST API.