`airshipctl config` doesn't yet support setting a context's
managementConfiguration value. This change adds in a sed
to set all managementConfigurations to default.
Change-Id: I0d8d0fb285a0b82e348640ef689eb3f0efb9e547
(cherry picked from commit bbd0ca28955c462077d05e2b77d18bfac8c47c45)