Implement api proxy for native charms

This patch reads the OSMLCM_VCA_APIPROXY env variable into vca_config,
and passes the new parameter to the N2VC client.

It also enables the deployment of native charms.

Change-Id: I28fb9167e2dc39b1534aeb1d27b86536d862f45c
Signed-off-by: Adam Israel <adam@adamisrael.com>