Rename `command` helper to `execute`

Since we have already changed the signature of this helper in a previous
commit, rename it to further emphasize the difference, as well as make
the method name more verb-like.

Change-Id: Ie768052999d69c03943020b90eaccce9f2ecdb8b
Reviewed-on: http://gerrit.causes.com/36001
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>