proxy_configuration $ipv6_enabled is now boolean

The $ipv6_enabled parameter of proxy_configuration was a quoted boolean.
This is not recommended by the puppet style guide, instead convert it to
an explicity boolean.

The various callers in protoproxy.pp have been updated as well as the
template which was checking equality with a string "'true'" instead of
boolean value 'true'.

Change-Id: Ib597a8a43674212cebbcf8ccbb916e7861cbf54e
Merge "proxy_configuration $ipv6_enabled is now boolean" into production