Support for nested entry objects

[kklimonda-cl]

Some objects are nested within other objects, e.g. interface can have multiple interfaces.

An example object xpath:

<request cmd="set" obj="/config/devices/entry[@name='localhost.localdomain']/template/entry[@name='codegen_template']/config/devices/entry[@name='localhost.localdomain']/network/interface/ethernet/entry[@name='ethernet1/2']/layer3/units/entry[@name='ethernet1/2.12']" />

This cannot be currently described in YAML spec, and cannot be properly rendered by codegen.

The following changes to the generated code are needed:

• In Pango, location.XpathWithEntryName signature has to change from (version.Number, string) to (version.Number, ...string) - we'll probably rename it also to location.XpathWithValues
• In Terraform, a parent attribute is needed that will be passed in CRUD functions along with the name.

In codegen YAML specs we'll probably need a new resource type, and a way to express full xpath suffix (with parent variable).
This is complicated due to the fact that nested entry objects may have different xpaths based on variant:

• ethernet subinterface xpath includes variant as parent xpath along with the name.