add support for using existing bridge as network

A network can now use an already existing bridge on the node.
Therefore, domains can now be directly connected on the same L2 Ethernet
network as the node itself.

A new optional bridge parameter has been added in YAML network
definition in order set the name of the existing bridge to use.

ConfigurationNetwork class has been improved to raise proper exceptions
instead of warning level logging when problems are encountered during
the parsing of the YAML structure.
ignore useless exception when error in __del__()
update the domain XML sample comment

This more close to what Cloubed generates for the network interfaces
of the domains.
add abstract generic ConfigurationItem class

This is the generic parent class to all Configuration* classes.
new method to parse ip_host and netmask in YAML

Added new method ConfigurationNetwork.__parse_ip_host_netmask() to
better parse ip_host and netmask parameter in YAML file.
add method to parse dhcp parameters in YAML

Added new method ConfigurationNetwork.__parse_dhcp() to better parse
DHCP parameters of networks in YAML.
add method to parse pxe parameters in YAML

Added new method ConfigurationNetwork.__parse_pxe() to better parse
PXE parameters of networks in YAML.
add method to parse names in YAML

Added new method ConfigurationItem.__parse_name() to better parse
all names parameters of networks YAML.
new method to parse testbed name in YAML

Added new method Configuration.__parse_testbed() to better parse the
testbed name in YAML
new method to parse items in YAML

Added new method Configuration.__parse_items() to better parse all the
different items (storage pools, storage volumes, networks and domains)
in YAML
new method to parse global templates in YAML

Added new method Configuration.__parse_templates() to better parse all
the global to testbed template variables in YAML