Add ping retries and break on success

Add ping retries and break on success

Change the ping command to take a retries value. This
allows the ping to be more independent of timing issues
with flow additions.

The cirros images and the SSHLibary have some weird quirks
that the patch had to workaround:

- The cirros images use a dash like shell and not bash. This
required catering the shell command to be posix compliant.
- The typical prompt of $ is in the shell command so it can't
be used to read until. This was changed to read until regexp
to look for the final rc=n from the command.
- The SSHLibrary Write keyword actually consumes and returns
the command itself, which was causing an early match. The regex
helped here also since the regex would only match on the rc=n -
so a number had to be in the output for it to match.

JIRA: NETVIRT-1399
Change-Id: I06d43fbd2ce7a0f32ccc60cdfd2783abaadd260b
Signed-off-by: Sam Hague <shague@redhat.com>