Change `cmd` parameter in `ExecuteProcess`

Feature request

Feature description

The current format of cmd is a little confusing. e.g.:

ExecuteProcess(['ls -las'])  # fails
ExecuteProcess(['ls -las'], shell=True)  # Ok
ExecuteProcess(['ls', '-las'])  # Ok
ExecuteProcess(['ls', '-las'], shell=True)  # Ok

That is maybe not of much interest, but the following case:

ExecuteProcess(['command', EnvironmentVariable('USER_ARGUMENTS')])

It's not way of specifying more than one argument in that environment variable, e.g.:

export USER_ARGUMENTS="-opt1 -opt2 -opt3"

That will make the program crash. The same problem applies for any substitution.
I don't see a possible workaround for that (without forcing shell=True).

My idea is to change the cmd type from:

cmd: Iterable[SomeSubstitutionsType]

to:

cmd: SomeSubstitutionsType

And use shlex.split after the substitutions are performed.

For backwards compatibility, we could accept both:

cmd: Union[SomeSubstitutionsType, Iterable[SomeSubstitutionsType]]

I realize about the problem while working in the launch frontend: https://github.com/ros2/launch/pull/226#discussion_r293036243.

Start with the ExecuteProcess entry point and the discussion in ros2/launch#226, which motivated this request. Trace how cmd substitutions are currently represented and expanded, then resolve the behavior for string, iterable, and shell-based commands. Done means the accepted API handles multi-argument substitutions without breaking the existing forms.