mgr/dashboard: fix nested shell quoting in cephadm e2e start-cluster

mgr/dashboard: fix nested shell quoting in cephadm e2e start-cluster

with_libvirt wraps commands in sg libvirt -c "$1", adding an extra
shell layer. Nested double quotes inside the outer double-quoted
string caused the argument to be split — with_libvirt received a
truncated $1, producing "Unterminated quoted string" on the remote
shell.

Drop the unnecessary inner double quotes around cephadm shell
arguments since cephadm shell accepts the command as separate args.
Use single quotes for the grep pattern inside the double-quoted
string so it survives the sg subshell.

Signed-off-by: Afreen Misbah <afreen@ibm.com>
(cherry picked from commit ae1fee05818e202287609ec1cbebd5f73ed4fd81)