Support env variable substitution with defaults in rebar shell

[richcarl]

The sys.config.src file may since OTP 21 contain env variables with defaults using the standard shell syntax ${FOO:-some_default}. The only place where Rebar itself needs to worry about this is in the shell command which wants to load the sys.config, and must then perform the substitutions first. This commit implements support for such env variable defaults. Previously, Rebar would crash if it encountered this syntax.

By some simple refactoring, this also makes the rebar3 ct --sys_config option work on sys.config.src files.

[richcarl]

Would be nice to get this merged - it allows you to write sys.config.src files like this:

 [{https_port, ${HTTPS_PORT:-8443}}
 , {https_cert_dir, "${CERT_DIR:-certs}"}

and get the variables replaced at build time if set, and with reasonable default values if not.

[ferd]

Should be good to go, but I'm proposing a bit of a simplification/encapsulation change to the parsing rule.

[ferd]

using ([a-zA-Z_]+[a-zA-Z0-9_]*)(:-([^}]*))?} for the regex would let you capture [1,3] and not have to handle the :- operator in the output, and simplify replace_varname/2.

[richcarl]

Good idea. When testing this, it turned out that the regex also needed to be left-anchored, otherwise it will match on the later part of a variable with a bad character, e.g. "HELLO-FERD}" would return a match on FERD as the variable name.