Add automatic build for ops charms

Added charmcraft dependency for building charms.
For using this utility, place the src code of the charm under
charms/ops, and the osmclient will build and place the charm code in the
charms/ folder.

Change-Id: Ib86d9ac966822131b2df3fc7621b42a49be7baba
Signed-off-by: David Garcia <david.garcia@canonical.com>