pct: add keep-env option to the 'enter' and 'exec' command

pct: add keep-env option to the 'enter' and 'exec' command

The keep-env option allows the user to define if the current
environment should be kept when running 'pct enter/exec'. pct will now
always set '--keep-env' or '--clear-env' when calling lxc-attach to
anticipate the upcoming change in default behavior.

Signed-off-by: Folke Gleumes <f.gleumes@proxmox.com>
 [ TL: fix some extra whitespace, extend subject slightly ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>