Improve OSU benchmark options

Here is a current valid campaign invoking osu:

benchmarks:
  \*:
    test:
     type: osu
     arguments:
       categories: [osu_bw, osu_latency]
       arguments:
         osu_bw: [-x, 200, -i, 100]
         osu_latency: [-x, 200, -i, 400]

This issue describes a set of changes to the arguments parameter of osu benchmark

rename arguments to options to remove confusion with arguments key at top-level. Previous campaign file should still be usable: If options is not set but arguments is, then hpcbench should raise a warning but still take arguments into account. for instance:
```
benchmarks:
  \*:
    test:
     type: osu
     arguments:
       categories: [osu_bw, osu_latency]
       options:
         osu_bw: [-x, 200, -i, 100]
         osu_latency: [-x, 200, -i, 400]
```
allow partial override of options (formally arguments) i.e user should not have to specify options of all categories when he only wants to customise one, for instance:
```
benchmarks:
  \*:
    test:
     type: osu
     arguments:
       categories: [osu_bw, osu_latency]
       options:
         osu_bw: [-x, 200, -i, 100]
```

if value is a list instead of a dict, then it is used for all categories, for instance:

benchmarks:
  \*:
    test:
     type: osu
     arguments:
       categories: [osu_bw, osu_latency]
       arguments: [-x, 200, -i, 100]

BlueBrain / hpcbench

Improve OSU benchmark options #284