Separate RHEL/CentOS network config from Debian/Ubuntu

Related to #15

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Parameterise network configuration over distro type

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
The path to whiptail varies by platform

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
The path to 'service' varies by platform

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Critical bugfixes

I need a typechecker

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Keep track of whether we actually need to rewrite /etc/network/interfaces or not

Signed-off-by: David Scott <dave.scott@eu.citrix.com>