Unify the naming convention of pre/post_run and pre/post_build_cmd commands

[victorusu]

ReFrame 2.x has an asymmetric naming convention for these variables

    prebuild_cmd = fields.TypedField('prebuild_cmd', typ.List[str])
    postbuild_cmd = fields.TypedField('postbuild_cmd', typ.List[str])

    pre_run = fields.TypedField('pre_run', typ.List[str])
    post_run = fields.TypedField('post_run', typ.List[str])

I propose that we unify them for ReFrame 3.x to

    prebuild_cmds = fields.TypedField('prebuild_cmd', typ.List[str])
    postbuild_cmds = fields.TypedField('postbuild_cmd', typ.List[str])

    prerun_cmds= fields.TypedField('prerun_cmd', typ.List[str])
    postrun_cmds = fields.TypedField('postrun_cmd', typ.List[str])

[vkarak]

Let's go for the _cmds variants everywhere.