Add a command line option for setting the verbosity level of the output

[vkarak]

Internal issue: https://madra.cscs.ch/scs/reframe/issues/126

[vkarak]

I am raising the priority of this issue, since oftentimes people wonder why their tests are skipped. On the other hand, for production runs, we don't want so much output. So we should compromise. This issue relates to #411.