(maint) Use Kernel.system for remote_ssh_cmd instead of ex

Because ex saves all output to a variable, commands that depend on user
input, such as pinentry/gpg-agent will not work as the screen will never
be rendered because the command hasn't completed yet. This commit
updates that call to use Kernel.system instead, which only returns true
or false and doesn't capture the output of the call. The current
implementation caused ships/signs to hang indefinitely during the
keychain passphrase section.
Merge pull request #314 from haus/maint/master/use_kernel.system_in_remote_ssh_cmd

(maint) Use Kernel.system for remote_ssh_cmd instead of ex
(maint) Migrate rsync_* into Pkg::Util::Net

This commit migrates the rsync_to and rsync_from methods from 00_utils
into Pkg::Util::Net. 00_utils now has deprecations for rsync_to and
rsync_from and will automatically dispatch to the new methods. The new
methods have an explicit parameter list, which was already being used in
all instances in packaging. As it is being moved from rake to pure ruby,
ex is used instead of sh in the invocation.
(maint) Use fully qualified paths for remote_ssh_cmd

Previously the remote_ssh_cmd utility shelled out to ssh when the fully
qualified path was available. This commit updates the implementation to
save the results of check tool and then use it. The tests are also
updated for this change and some stubs are made more specific.
(maint) Update all instances of rsync_to to Pkg::Util::Net.rsync_to
(maint) Update all instances of rsync_from to Pkg::Util::Net.rsync_from