junos: support edit/top keywords in set based configs

[ckishimo]

This PR adds support for edit and top keywords in set based configuration in Junos. The edit is used instead of set if one needs to add comments using the annotate statement as you need to be at the right hierarchy level

For instance:

top edit system ntp
annotate peer 1.1.1.1 "Test ntp server"

system {
    ntp {
        /* Test ntp server */
        peer 1.1.1.1;
        peer 2.2.2.2;
    }
}

One cannot add a comment with a single set command

[coveralls]

Coverage remained the same at 78.967% when pulling 79bfbcd on ckishimo:edit into c5e5f60 on napalm-automation:develop.