create generalized function for ssh-client

[rsmekala]

Cherry-picked code from #896. Cannot merge it due to merge conflicts

• Added generalized script for ssh_client open utility
• start_shell and scp modified to use the generalized utility.

Test logs

Script

from jnpr.junos.device import Device
from jnpr.junos.utils.start_shell import StartShell
from jnpr.junos.utils.scp import SCP


with Device(host='xx.xx.xx.xx', user='xxxxx', password='xxxxx') as dev:
    with StartShell(dev) as shell:
        result = shell.run("ls /var/tmp/NSSULogs.pdf")
        print(result[1])

    with SCP(dev) as scp:
        scp.put(files='bin/NSSULogs.pdf', remote_path='/var/tmp')

    with StartShell(dev) as shell:
        result = shell.run("ls /var/tmp/NSSULogs.pdf")
        print(result[1])

Output

ls /var/tmp/NSSULogs.pdf
ls: /var/tmp/NSSULogs.pdf: No such file or directory
% 
ls /var/tmp/NSSULogs.pdf
/var/tmp/NSSULogs.pdf
% 

Process finished with exit code 0

[jnpr-community-netdev]

Can one of the admins verify this patch?

[coveralls]

Coverage remained the same at 94.885% when pulling ff36677 on rsmekala:ssh_open_utility into 96b8c2d on Juniper:master.

[vnitinv]

looks good