Add netconf.api.CapabilityURN

Add netconf.api.CapabilityURN

We have NETCONF capability strings strewn all over the codebase, in
various forms and at various layers. Centralize the definitions based
on the current contents of the IANA registry (plus the EXI part, which
is just proposed).

Centralize the use of the constants, eliminating a number of
XmlNetconfConstants, i.e. disconnecting code from the XML encoding to a
certain extent.

JIRA: NETCONF-1017
Change-Id: Id03708c24d48fd65547f294827d8c15d9d7dd6e7
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>