Add multi-SCM capabilities

Jenkins has a multiple-SCM option. We can just support that by using it when
more than one SCM module is defined. Simple enough.

Change-Id: Iacfba5c547ff111304644a4777a12a70b0b22e62
Reviewed-on: https://review.openstack.org/23961
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Antoine Musso <hashar@free.fr>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
`update` can now takes several job names

Just like `delete` can accept several jobs names as argument, this patch
let us do the same with `update`.

* builder.update_jobs now filter the jobs against a list
* renamed 'update' argument 'name' to 'names' to make it clear it
  accepts several job names.

Change-Id: I4bcebd98073ec89e172afbec59b93d14de6620ff
Reviewed-on: https://review.openstack.org/24160
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
doc/configuration: Fix incorrect name "defaults"

See YamlParser#applyDefaults in builder.py.

Change-Id: I763d23608b79f7b1c560ae41cf5ca0d2dcc19d60
Reviewed-on: https://review.openstack.org/24407
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
'names' argument to update no longer required

Before the change to allow 'update' to take multiple jobs the
'names' argument was not required and passing just the yaml file
would create all the jobs inside it.  The 'update' change made
names required when it shouldn't be.  This is a one line change
to cmd.py to change nargs from '+' back to '?'

Change-Id: I65372d78ec678b9c29caad2ea573772287a7642f
Reviewed-on: https://review.openstack.org/24673
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins