system prune: only warn for volumes if --volumes is given

[albers]

#118 changed docker system prune to only remove volumes if --volumes is given.
The warning message, however, still always warns about volume removal:

$ docker system prune
WARNING! This will remove:
        - all stopped containers
        - all volumes not used by at least one container
        - all networks not used by at least one container
        - all dangling images
        - all build cache
Are you sure you want to continue? [y/N]

The warning message in the above example is wrong: no volumes will be deleted.

With this PR, the volume removal warning is included selectively depending on whether --volumes is given or not.

[codecov-io]

Codecov Report

Merging #255 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
+ Coverage   46.84%   46.85%   +<.01%     
==========================================
  Files         172      172              
  Lines       11692    11692              
==========================================
+ Hits         5477     5478       +1     
+ Misses       5903     5902       -1     
  Partials      312      312

[mlaventure]

I know it's only for 2 lines, but I think a text template would be cleaner.

Something like: https://play.golang.org/p/nLs0YvRrRE

[albers]

@mlaventure Thanks for your advice and the code sample. I pulled out message generation into a function that uses a template. PTAL.

[dnephin]

LGTM

[mlaventure]

LGTM