Set common rsync and dsh parameters in mw-deployment-vars

scap-2 and sync-common-file each exclude a different set of file-patterns from
rsync. sync-common-file excludes '.svn' and '.git' in their entirety, whereas
scap-2 only excludes git objects and SVN lock files. dsh invocations are
consistent, but the same set of of command-line arguments is specified in five
different places. This patch centralizes common arguments to dsh and rsync in
mw-deployment-vars.

Because some of these scripts run on scap targets too, it's necessary for
mw-deployment-vars to be deployed to all scap targets. This is made somewhat
complicated by the fact that the 'misc::deployment::vars' class that generates
that file takes a parameter, '$system', which defaults to 'git-deploy'. Because
we need to invoke the class with a different value for $system (namely, 'scap')
we can't simply "include misc::deployment::vars". We have to use the resource
definition idiom ("class { 'foo': param => 'paramvalue' }"). The problem there
is that while a class may be included any number of times (and the includes may
be cyclic), the resource-style class declaration can only be used once per
class.

To work around this, I changed the default value to 'scap', which allows both
"misc::deployment::scap_scripts" and "mediawiki::sync" to include the vars
class. I then changed "misc::deployment::scripts" (used by git-deploy) to use
the resource declaration idiom, so it can specify the (now-non-default)
"git-deploy" parameter.

Bug: 40025
Change-Id: I702777b4ada8706b65322af45fcc2f402aad54db