cmd/juju: update docs with info on placement directives

Also, bring add-machine docs up-to-date and a bit more cohesive.

Fixes https://bugs.launchpad.net/juju-core/+bug/1387421
cmd/juju: permit user@ in scp/ssh target

The "juju ssh" and "juju scp" commands now
allow "user@" in the target. For example,
you can now do "juju ssh jenkins@jenkins/0"
to ssh to the first jenkins/0 unit as the
jenkins user.

Also, fix examples and clarify help for
"juju scp" regarding passing additional
arguments to "scp".

Fixes https://bugs.launchpad.net/juju-core/+bug/1387766
Fixes https://bugs.launchpad.net/juju-core/+bug/1387640
Fix typos
Merge pull request #1013 from axw/lp1387421-juju-help-placement

cmd/juju: update docs with info on placement directives

Also, bring add-machine docs up-to-date and a bit more cohesive.

Fixes https://bugs.launchpad.net/juju-core/+bug/1387421
provider/azure: make boilerplate <> usage consistent

Remove <> from boilerplate values for consistency's
sake, to reduce confusion about what needs replacing.
This matches the docs.

Fixes https://bugs.launchpad.net/juju-core/+bug/1381289
Merge pull request #1014 from axw/lp1387766-juju-scp-username

cmd/juju: permit user@ in scp/ssh target

The "juju ssh" and "juju scp" commands now
allow "user@" in the target. For example,
you can now do "juju ssh jenkins@jenkins/0"
to ssh to the first jenkins/0 unit as the
jenkins user.

Also, fix examples and clarify help for
"juju scp" regarding passing additional
arguments to "scp".

Fixes https://bugs.launchpad.net/juju-core/+bug/1387766
Fixes https://bugs.launchpad.net/juju-core/+bug/1387640
Merge pull request #1015 from axw/lp1381289-azure-boilerplate-consistency

provider/azure: make boilerplate <> usage consistent

Remove <> from boilerplate values for consistency's
sake, to reduce confusion about what needs replacing.
This matches the docs.

Fixes https://bugs.launchpad.net/juju-core/+bug/1381289
Make add-metric test independent of map ordering.
Allow empty values in juju set-env.
Merge pull request #1020 from tasdomas/fix-1388073

Fix 1388073 and the worker/uniter/jujuc test failure in 1387936

Command line parameters for juju set-env were being parsed incorrectly, not allowing empty values ('key=') - that was fixed and a test added to avoid future regressions.

Tests for worker/uniter/jujuc were failing on PPC due to reliance on map ordering, which is not guaranteed. Fixed.